Raimey,
The 9x56 on the other flat was half visible in the photo and if there was a date code under it, the date and ledger number would have been partially visible.
I didn't "make the jump" from 9x56R to 9x57R; the owner of the rifle identified it as 9x57R. I usually accept the owner's ID of the caliber, unless he says he isn't sure. Of course, sometimes I get bad info to start with and get "wrapped around the axle", but until that happens, I have to accept his info as being correct. If it turns out to not be a 9x57R, we can address that then. The markings on the barrel don't represent the nominal caliber of the rifle, until the 1939 proof law came into use.
Mike