Todd,
Pretty sure I have this starter on both the 521 and the 557:
https://www.rockauto.com/en/moreinfo...sn=491&jsn=491
I don't know where they came from but they were supposed to be OEM for a 1996 F250 7.5; in other words a stock Ford truck starter. That seems to be where the 460Ford guys ended up with their strokers.
All the fancy aftermarket starters I tried failed, but not in the same way you described yours...
I wonder if yours just got tired and couldn't pull all the way in?
Phil, I think you can see from the illustrations that the pinion seems to be well supported.
Tom