Wild_Bill Inscribed thus:

There doesn't seem to be a good reason for the pins. Maybe the only
way to find out what problem (real or imagined) Rockwell designers
thought they were addressing, would be to find some advertising
literature from the time when this type of "feature" was introduced.

Putting a thru hole that size in a 1/2" shaft doesn't seem reasonable
to me, but I assume someone thought it would be a good idea.

If there were wrench flats (or pin spanner holes) on the inner wheel
flanges to fit a wrench when removing and replacing the shaft nuts, I
could see some perceived practical use for the pins, but then only
really useful if the nuts were nylock types (any type of locking nut
is not actually required for mounting grinding wheels), because
grinding wheel nuts don't require significant torque to make the
mounting secure.

I've seen used bench grinder arbor threads that have been chewed up by
someone using a plier on them, apparently by someone not smart enough
to figure out the right direction to turn the nuts, or not confident
in having the nuts just snug enough to firmly secure the grinding
wheel.

I've also seen wheels with significant gouges in them, so if an
accidental jam on one side caused the motor's rotor to stop, it's
possible the other wheel could continue to rotate, but that's still
not likely to unscrew the nut from the arbor threads.

Isn't it possible that the idea is to hold the stone at one end whilst
tightening the nut at the other. A cross pin would achieve that aim.

--
Best Regards:
Baron.