O.K. I finally figured out what this is, thanks to the Mauser Message Board
This is NOT a Mauser rifle, technically, but a "guild" rifle.
A Guild rifle is essentially a custom rifle assembled by a member of the Gunmaker's guild.
To my mind that tends to explain the modest engraving on the receiver and baseplate.
The basis of the rifle is a Mauser Commercial action/receiver/barrel, which typically
bears proofmarks under the barrel near the receiver.
Changing rules for proofing marks make the following deductions tentative.
Particularly frustrating is the number of stampings with faint edges.
At top, N with crown: standard on "nitro" rifles.
7,8 (and the left edge of a 5?) with crown above indicating the caliber 7.8mm = 8x57J.
The next number "57" likely indicates it is the 57th rifle proofed that year.
535 may indicate a proofing date of June 1935.
More guesswork to be added at a later date