It isn't strange. Just look at other guns that were proofed in Munich like this ERMA:From 1977 onwards, the year of the proof test was always coded, as on the left side of the frame with HH = 77, 1977. Behind the alleged sign of the Munich proof house, however, 77 is a number and not a code. Strange, isn't it?
Excerpt from the "Allgemeine Verordnung zum Beschussgesetz (Beschussverordnung - BeschussV)":