[MJN]

When I rebuilt my SM420 the only thing that resembled a roll pin was the small pin that held the reverse idler gear shaft in place, #57 in the exploded view in the pdf linked above. In order to remove the reverse idler gear shaft you have to drive that pin into the idler gear shaft (in the linked pdf above it is step 13 on the second page). It's possible an old one fell into the case during a rebuild in the past.