A&A will have them. Like Locomotion said, they're mostly found in '78-up 904s but not all units have them. The info below is cut and pasted from this thread;
Differences in 904 sun shells
Something to be aware of is the later ('78-up) low gear sun shells are different than earlier models. You have to get
everything that goes with it to make it work - the planetary set, the shell, sun gear, snap rings and bearings, etc.
The low gear shell is on the left. See the step at the bottom? The step accommodates a spacer used with the 'low gear' planetary. You have to have that piece or it won't work.
View attachment 1716206726
I believe
all 904 sun gears are the same except the low gear one uses this machined spacer. I don't know why they went with this setup, maybe has to do with the lockup feature on the later units. The lockup units have different input shafts and I think the overall stacking tolerances might be slightly different. Either way, you can definitely use the low gears in an earlier unit but again, you need all the parts.
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