From the 1939 Stoeger:

In English Measure:

Here is fresh, 8X56 headstamped Qual Cart brass: Midway

If your MS is original and has not been 'rebored', it is an 8X56 MS. If you have any doubt about its originality, you may wish to have a cerrosafe cast made of your chamber to be sure.

Once you've established that, Reed's Ammo of Oklahoma makes excellent reloads Reeds
that will be the proper profile for your 8X56MS Schoenauer magazine. They can be rather particular as to what they're fed.