As stated the original M&P's were made by CMT and then assembled in house by S&W. I don't know that there is much to "prototype" with a standard AR15 lower, so I would be suspicious of such a claim.

On the other hand, it could be a blemished lower. They milled off the logo's and sold it to recoup some small amount of profit.

Are there any marks on it that say who made it? There should at least be a manufacturer name, location, and serial number.