Wellll, it certainly is possible that whoever stamped that 248 intended it to be a gauge mark, but it wasn't an "official" proof law standard to do so. The gauge marks from .300" to .450" in .010" increments were established by the 1868 British proof law which Germany included in their first proof law of 1891. There's a table of 'em here at the bottom of the page. Gauge marks other than these within that diameter range would not be likely from the proof house.

A bore of 248 gauge, at .266-.267", seems pretty tight for a 7mm with a nominal .284" groove diameter, but those German gunsmiths did lean toward tight barrels.

2-piper, cube roots to six places really does beat TV!!