Greg E.-
The output spline is the same for all standard-frame GT pumps. The main difference I can think of is the motor swash plate; the design of the later one is a little different. In the later one, it is cast into the housing, and the slippers ride on a big washer, whereas in the earlier one, the motor swash plate is this much larger iron casting that bolts onto the inside of the case.
Charlie/Jim-
I haven't taken apart my two aluminum ones yet, but it appears that the main shaft, planet carrier, planet gears, shift collar, etc. are the same. I know for sure that the shift lever is different, as is the detent mechanism.