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!!