"On the contrary, the three I played were obviously carefully built guitars: after all, they would be scrutinized to the hilt if it were to make it to production, right?"
Well, I guess that may be true but I'd be concerned that since it's a prototype, maybe that specific version DIDN"T make it into production. IOW, maybe the production version corrected issues the factory found in the prototype after constructing/playing it.
OTOH, I would think that if they then labeled/badged it and sent it out for sale, it should be, as you noted, perfectly OK. But obviously there is some difference between that prototype and the standard prod model; otherwise it wouldn't be labeled a prototype.
Could it be the wood type? IOW, if the body or top is made of a different wood than the prod model, could that make it a proto?
It's not that big a deal - I will be looking at plenty of guitars in a couple of weeks but I found it interesting/confusing when my friend told me about the "prototype."