Looks like you got a nice tractor mounted mixer. Congratulations!
The barrel looks similar to the Prewitt. I'm not sure, but believe Prewitt bought the barrels to assemble on frames they made to fit various tractors. I've seen other barrels that were identical to what Prewitt used, but stand-alone.
Other Prewitt photos and brochures I've seen has a different shaped cast iron support beam at the rear of the barrel (looks like an I-beam in cross section), whereas this one appears to be round, tubular steel. This one also has the large pulley wheel extended out away from the barrel; believe the Prewitt was fairly close to the frame and barrel. Frame is just a little different too.
Again, congrats!! Looks to be in really great shape!!
Mixer looks to be in good shape. I think mixers "all look the same" because how many ways are there to make one. It's sort of like old horse drawn hay rakes, everybody and their brother, not just the big boys, made those and they all look basically the same because when you get right down to it they all work the same. You have to look real hard to find differences.