IMHO, If the car can be independently documented as an engineering prototype R3 and all the original parts are there, then $15k is a bargain. However, until one of our experts has done the due diligence, it's a crap shoot.
We couldn't count the similar South Bend legends, such as the "factory-supercharged '59 Larks", the '56 Golden Hawks with 2x4bbls from the factory", "the special-ordered C-cab pickups with a supercharger and 4-speed" and "Avanti R4s" and many others which have come through here over the years.
Until proven, The Legend of the Lost R3 Prototype" is just that.
jack vines