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Sorry the cross section got inserted twice and sorry the picture is small. The photo I have in my file can be enlarged. Let me know if you do not have access to a larger picture and I will try another route.
Looking at this cross section, I do not see a seal ring at the front (left) end of the output shaft. I see several retaining rings, one spacer ring, and probably a thin thrust bearing ring between the sun gear and the ball bearing’s inner race.
Near this end of the output shaft, I see a retainer ring in a groove holding two clutch pack assemblies onto the output shaft. I see a spacer ring between this retainer ring and the ball bearing.
I see a groove in the OD of the ball bearing with a retainer ring in this groove. The ball bearing and its retainer ring are held in the planet carrier by another groove and another retainer ring.
In the lower ranges and reverse, there is a high rotation speed difference between the sun gear on the input shaft and the output shaft holding the ball bearing’s inner race. Lubrication at that interface seems sparse so a thin bronze thrust bearing would be in order to prevent galling.
The assembly process seems to be to first assemble two clutch pack assemblies onto the output shaft as you mentioned having done. Add the spacer ring to this subassembly.
Another subassembly on the bench would be the planet carrier with its gears, the ball bearing attached to the planet carrier with the two retainer rings, and the thin bronze thrust bearing (if any) between the sun gear and the ball bearing. Install the input shaft in this subassembly to hold the bronze thrust bearing in place.
Assemble the two subassemblies to where the inner race of the ball bearing contacts the spacer ring placed on the output shaft. At the rear, add the thrust bearing and retainer ring noted as number 12 and 13.
I have never paid close attention to one of these transmissions apart so if my information is amiss, please let me know.
The input shaft supports the input sun gear. In turn, this sun gear’s three meshes support the planet carrier and ball bearing. In turn, the ball bearing supports the front of the output shaft. In turn, the front portion of the clutch pack assembly is supported by the ball bearing. This is known as cascading support and tends to be undesirable. Case was wise to discontinue this design and later go with the IH power shift in the merger of CaseIH.