Masters and instructors, I banked on your combined theories and ordered a new drive block in advance.
After much overtime on my "day job" I finally got the time to work on this. Replaced drive block last night.
Interestingly, it appeared that the tabs on the drive shaft were more worn than the drive block, but the new block fit very tightly and preliminary test runs have been successful!!
You guys are awesome, thank you all.
Thank you Fermented One for chiming in.
Durham, you were right in that Kurtiusinteruptus guessed it first, blindfolded.
Thanks again, all.