My guess is they just took the TC10 allison and shrunk it for light duty use. They probably changed the control system to fully hydraulic too.
Obviously just a guess, but makes sense that they would use existing tech instead of a total start-from-scratch approach.
1993 K3500- 4x4, single cab/long bed, single rear wheels, 4.10 rear end, F vin, Nv4500 manual, factory red interior.
Gone but not forgotten: 2001 silverado 2500hd CC/SB, 4x4, 6.0 vortec, 229k miles when I traded it.