i have a pro tl body... the rewind crank isn't at all wobbly. because of the shutter speed dial there is only one way to get the winder or the crank on, so it is really foolproof, (thank god).
with the 645e that doesn't seem to be the case - is it possible it's not seated properly? did you ever use it - maybe it's for another 645? is it the locking mechanism, or the winder? since the 645es was a "specialized package" from mamiya (see ken rockwell site) is it possible it had a unique mechanism?
i'm guessing and some thoughts that i'm sure you've had... but might lead you somewhere...