Let me start by saying I've never filed a form 1, so I've never had one kicked back for a lack of required info (like plans or specs). I've also never heard of this requirement on a form 1. When I look at the form 1 where it asks for plans & specs if its a "prototype", I think they're getting at the situation where you make a complete new weapon type and want to designate it with a model designation that wouldn't be known or would likely be mistaken for something else. For example, if you put MG34 on your form 1, the people at BATFE know what an MG34 looks like and would expect what you built to generally look like an MG34. OTOH, if you put "The Haven-ator" down as the model instead of MG34, they wouldn't know what a "Haven-ator" is suppossed to be or look like. Similarly, if you wanted to call your new MG34-like creation a "PPSh41" on the form 1 (just because you wanted to be different), they'd be thinking what you made was something resembling a PPSh41, not something that really looked like an MG34. I think thats the problem they're trying to address on the form 1 when they ask for plans if its a prototype. BATFE knows what a suppressor is, and from the rest of the form they know the caliber and size. I don't think the Tech Branch is looking for every individual filing a form 1 for a can to include a set of plans to build a traditional k-type baffle can or a neoprene wiped can. That wouldn't be a prototype in my opinion; it would just be another k-baffle can.
I see the logic though - that anything an individual wanted to make would be a "prototype" for that individual insofar as he was the "manufacturer" on the forms. I just don't think thats necessary or required if its a can or if not a completely new and different firearm type; like a "Haven-ator" if it was a firearm. It not, almost everything anyone made on a form 1 would require plans & specs (isn't the F2 for manufacturers?). But again, I could be quite wrong; as I said I've never filed a form 1 so I've never had one sent back requesting plans and specs. In 12+ years of dabbling in NFA, however, I've never heard of anyone getting a form 1 for a can (or anything else) getting kicked out for not including plans.