There isn't much more that I can help with unless you have more and better photographs of the additional numbers and proof marks. What you would be looking for are maybe the letters B, U, G, N with crowns over the tops of each and the serial number. These were used between 1924 and 1934 with some carrying on through the later years.
B = rifle proofed in assembled form.
U = final inspection proof.
G = sporting firearm with rifled barrel.
N = indicates Nitro proofing. (from 1933-1939) Also used on military contract rifles for export.
Non-exported receivers had a serial number on the left side of the receiver above the stock line. Exported receivers had the serial number on the bottom of the receiver so that the maker could assign their number to the side.
The caliber could have been placed on the left side of the barrel or underneath on the barrel. Along with this number underneath was the caliber markings: land and groove size (mm) as well as the twist rate (mm) and possibly even the steel maker's name.
The ballistics established by the Mauser factory for the 6.5x68S was:
Spitzer bullet weight 127 gr.
3,130 FPS. at the muzzle.
2,795 Ft/lbs. at the muzzle.
56,000 PSI. chamber pressure.
If you want to get a thorough grounding in Mausers, I would suggest buying the Ludwig Olson book and the Jon Speed book on Mauser sporting rifles.
I don't recommend tapping the rifle anywhere on it as it will ruin the potential value. As has been pointed out, the stock is not suitable for scope mounts, anyway. It was done but the cheek weld was non-existent.
If you can take the rifle outside and photograph the numbers and proof marks, I might be able to help more. If not, I think that this is the best that I can do.
Best of luck with a wonderful rifle!
Edited by nononsense (12/11/04)