I'm not sure I understand which you're asking: when the stripped lower becomes a firearm, or whether building it as one form or the other prevents it from being built as any other type.
Manufacturers may build lowers with one intent or another and list them that way, but AFAIK the only thing you have to worry about is running afoul of the laws that prevent various unlawful configurations of this stock/that barrel, etc.
As long as you don't have a "stock" on your "pistol" (as referred to above) you really don't have anything to worry about.
Note that the form 4473 only has to do with the gun as sold, or in the case of a receiver, what type of receiver it is (rifle/pistol). In the case of a stripped receiver, or more to the point,
frame, the only way to determine whether you're legal to own it is to run the check as appropriate for that type of firearm. What you do with it after you own it is governed by other laws, such as the NFA, etc.