A Fastback is specifically the type of body I posted a picture of above- it is the type made by Doc's Machine. It is a lot like Martins, Turtles, the new PBX bodies, Catapults, and those cut-top Osiris bodies, but it is a particular "brand." A Martin isn't a Fastback, a Martin isn't an Osiris. The ram sleeve in the bottom of the receiver body is what makes an Osiris unique. The shorten top part is becoming popular, but it is used by a few different airsmiths/ custom shops. The idea is to reduce the moving mass, for faster cycling.
I would be very interested to see Eclipse take up the cut-top type design. I think they have consistently made the best looking cockers for some time. I really like the shorten top tube, though. I think Fastbacks are the most attractive, but I guess that's all subjective.
