The parts list for Spicer Universal Joint for the 1927 Star lists a Cap Screw Seat P/N 22508 but does not show a picture of the part and I did not have a part that looked like it might serve this purpose.
The splines in the rear hub of the Spicer Universal Joint do not go all the way through the hub as the splines in the front hub do. The front hub is therefore free to move on the clutch shaft to allow the clutch plate to move laterally. The rear hub has a recessed hole on the front side with an ID of 1.016" which slightly larger than the 1" OD of transmission splined shaft. It is 1/2" deep. The shaft has a 3/8-24 threaded hole in the end of the shaft. The cap screw is 3/8-24 x 1" long. I knew the cap screw and the lock washer were used to secure the front hub the to the shaft to prevent the universal joint from moving forward into the throw out bearing fork but how?
I tried a flat washer that had a diameter larger than the recessed hole and secured it with the cap screw. This held the hub in place but the protruding head made it very difficult to install the spool.
My next try was to locate a washer that had an OD slightly less than the ID of the recessed hole (1.016") and is about the 1/2" deep. I found one that was .999". I screwed in the cap screw with the lock nut between the head and the washer. This seemed to work fine as the head of the cap screw was almost completely in the recessed hole and the spool could easily be inserted between the two hubs.
I felt this would work but still wasnt satisfied that it was correct. For some reason, I decided to dig through my collection of flat washers again. I found a second washer that was about the same diameter as the one I was using that I had probably overlooked because it had a scored area on one side that appeared to have been caused by a lock washer. I looked on the reverse side and I could see little rectangular impressions around the outer circumference. The impressions that the end of a splined shaft could have made when the bolt was tighten.
This convinced me that the Cap Screw Seat was nothing more than a flat washer that would fit snugly in the recess hole. How and when the old washer got in my collection of washers I dont know. The newer washer which has an OD of .999", an ID of 13/32" and is .136" thick. The old washer has an OD of .978", an ID of 13/32" and is .133" thick. I am using the newer washer as it has a larger OD.
Jan Arnett said on earlier units, a tapered pin was used to hold the hub in place.