I assume there was some reason to set back the shoulder a little bit in the 7x57.
Maybe to prevent accidental chambering of an 8x57 J cartridge? Or a 7,65 Argentinian cartridge?

Such an 8x57 /.308 zwitter was discussed intently in a German forum some months ago:
https://forum.wildundhund.de/threads/7-62-xxx-idee-meinungen.112386/

Certainly this works and may be fun, but the actual need and sense (except creating your own wildcat) escapes me.

fuhrmann