"COTW" contains many mistakes, especially regarding German cartridges.
The Dynamit Nobel (former RWS) reloading book "Wiederladen" writes about the 7x75R SE v.H.:
"The cartridge was announced by Walter Gehmann (who had taken over the vom Hofe firm in 1955) in the year 1958, however Ernst August v. Hofe had already experimented with the cartridge shortly before the war and he also brought ammunition into trade."

If the chamber of that particular rifle has been reamed from 7x65R to 7x75R at a later time, in Germany the rifle would have been subject to a re-proof, and should show the according proof marks.

Regards,
fuhrmann