G. van Vlimmeren, what you are seeing in the left side photo is a smooth spot in the otherwise sandblasted appearance of the side rail. It would appear that something had rubbed there and smoothed it out, perhaps the holster. Certainly not the marks left by a torch melting out the solder to remove a brass plate.
http://forum.lugerforum.com/lfupload..._side_rail.jpg
Martens and de Vries speculate about the little stamping of the letter 'C" over two digits. "Some parts of the Vickers guns have small marks consisting of the letter "C" over two digits." "The meaning of these has as yet not been satisfactorily explained. The "C" has been taken to mean "Construction", indicating the year in which the part was made. However the numbers run as high as 24, and in 1924 the Dutch contract had long since been fulfilled." This particular example has two "C" stamps on the frame, one being C/15 and the other C/40. This would appear to invalidate that theory.