Potter's reading of Groundwork I works by analysing the argument into nine propositions, on each of which he comments. The first one is his statement of the first sentence which Potter translates as stating that only a good will has "absolute" worth. What Kant writes is that only the good will has "unlimited" worth with the point of the first paragraph being that other things that one could call "good" are limited in their goodness whereas the "good will" is not so limited. Potter, rather oddly, in translating the statement of the opening sentence into a claim about "absolute" worth, gives himself an unnecessary puzzle concerning where Kant provides an argument for this special status. Kant doesn't have an argument for this status because he doesn't make a claim about it. Nor does he simply think this claim would be "accepted" by anyone as Potter also suggests since the argument is immediately given about the limited goodness of other good things which are thereby contrasted with the unlimited goodness of the "good will".
Potter also speaks of the "good will" as a possibly pre-analytic concept but whilst it is true that Kant does not initially analyse it the subsequent discussion (which turns into an account of duty) does show what the "good will" has to involve. So the first parts of Potter's account are not very perspicuous and he has to restate the first proposition of the Groundwork as concerning the relationship between different kinds of value claiming that it amounts to a statement that moral value outweighs other forms of value. However, this is still an odd way to read Kant's argument since virtues, with which the "good will" is contrasted in the second paragraph, are also usually viewed as having "moral value" so Kant's claim can't simply be taken to be one about the incomparable weight of moral value simpliciter. Rather a point is being made here about the nature of moral value that requires subordination of the 'virtues' in some way (albeit prior to critically indicating a new sense of their status later).
Potter's second proposition is better stating, as it does, that a good will is not made good through what it brings about, a point clearly made as Potter correctly points out, at the beginning of the third paragraph of the first section. Here Potter is on solid ground pointing to the derivative character of that which is good as a means compared to that which is good in itself. Since, however, Potter confused the first proposition, he aligns this claim about something being good in itself with possession of an "absolute" rather than an "unlimited" status.
Potter now misses entirely the discussion of teleology that Kant engages in after making the point about the derivative character of things whose "effect" is good. Potter misses this part of Kant's argument out of his reconstruction because, as he puts it later, it is "unusual and not very convincing". The reason why Kant includes it, however, is to give support for the claim of the unconditional goodness of the good will in abstraction from considerations of utility. (It is also at this point that Kant does mention the "absolute worth" of the good will, absolute by contrast "in worth" to all derivative forms of "worth". ) The argument is a pre-emptive one concerning the difference between the ends of reason and the ends of nature with the latter understood to involve a concern with "happiness" and in arguing for a disconnect between these ends it is important.
In skipping past the two pages of discussion of teleology Potter moves directly to the claim that human actions are, as he puts it, "morally good" if and only if they are done from duty. This touches on Kant's distinction between actions done "in accordance" with duty as opposed to being done "from duty". The relationship between these claims about duty and the earlier ones about the "good will" are, however missed if we don't attend to Kant's claim that duty "contains" the thought of the good will albeit under certain "limitations". Potter thinks of the claim about "moral goodness" as being one Kant thought to "follow from" the earlier claim about the "good will" but is unclear how it is supposed to "follow". Potter emphasizes that looking at duty means discussing the value of actions that follow from the will rather than focusing on the value of the will alone. What Potter doesn't mention, however, is that we require here an account of "character", one that Kant in fact gives in describing dutiful action, a view that is contrasted directly there with an account of "temperament".
Potter's fourth proposition concerns a further development of the notion of dutiful action which is related now to the maxim on which it is based. This point is described by Kant himself as a "second proposition" of his argument at Ak 4: 399 and is the source of the general view that Kant takes motivation to be central to moral worth rather than consequentialist appraisal. Potter, however, states that Kant is not here "entirely clear about what he is arguing for". The principle of volition is described by Kant here as 'formal' and the point of this is to rule out motives grounded on "desires" in the Humean sense as all such would be merely "material". So what does Potter find "unclear" in Kant's argument?
Well the principal claim that Potter presented as proposition four of Kant's argument is now analysed by him as containing two sub-claims. One of these claims is that the moral value of an action is a quality of its maxim. This point is assumed to have a status separate from the claim that the maxim is one that will have a formal quality. However whilst it is true that maxims need not be formal in quality the whole point of Kant's argument at this point is to stress formality in opposition to materiality and the value of a maxim that was not formal would be merely derivative as follows from the previous argument on Potter's own construal so this point hardly seems sufficient to suggest that Kant was "not entirely clear"!
The next proposition on Potter's account is that the formal principle of volition is the same as the requirement that actions conform to universal law. Part of the problem with transitioning to this point next is that Potter has abstracted in his reconstruction from the account of "respect" that Kant includes at this point as when he states that duty is the "necessity of an action from respect for the law". Kant makes here an extended argument distinguishing between "objects","inclinations" and "effects" to arrive at the point that there is something that can serve the will as its ground rather than being an effect of it. This is the point about the law. It is only after this argument has been made that Kant gives the first section version of the categorical imperative.
However Potter leaves this aside at this point of his analysis moving swiftly from a claim that the formal principle of volition should be the requirement of conformity to universal law to a suggestion that Kant views the "essence" of law to consist in universality. This would appear to be a metaphysical claim that would be odd in the context of Groundwork I and there is little sign that Kant made it. It is after introducing it that Potter then moves to the "seventh" proposition which he takes to be the version of the categorical imperative made in this section. Potter in fact refines the version of the categorical imperative stated in Groundwork I into a second formulation that translates acting dutifully into acting in accord with a maxim that can be willed as a universal law. Finally, Potter returns to the "good will" claiming that such a will would always act on maxims that can be willed as universal law.
Potter's last two propositions are not directly stated in Groundwork I at all and hence should have been marked by him as implications of its argument rather than being part of its reconstruction. Further, some key elements of the argument have been entirely missed by Potter. Potter has not included the discussion of the example that Kant mentions in Groundwork I of false promising or the way it is used to make a first pass at a contradiction in conception test. Nor does Potter mention the notion of "standard of judgment" that Kant refers to as something that ordinary moral cognition is supposed to have before its eyes. Finally, Potter only mentions the "natural dialectic" Kant claims belongs to such ordinary consciousness but makes nothing of this. Potter's reconstruction of the central argument of Groundwork I thus seems to me incomplete even without looking further at his discussion of philosophical disputes concerning the meaning of its argument.