The bore/land diameter marked with the proofmarks was established by the proofhouse using cylindrical plugs in .1 mm steps. So “6.3 mm” means: a 6.3 mm plug passed the bore while a 6.4 mm one did not. So 6.3 mm was marked on barrels up to 6.399 mm. Minimum bore diameters and chamber dimensions for the various calibers were set in 1925 only, several years after this rifle was made. The actual bullet diameter also depends on the groove depth. The marking here, 6.3 mm / 53,5 mm case length points to either the 6.5x54 K Mauser, 6.64 mm = .261” bullets, or the 6.5x54 Mannlicher-Schoenauer, 6.70 mm = .264” bullets. A chamber cast is advisable here.
The rifle was proofed by the Suhl proofhouse after 1911. It was built by V.C. Schilling, Suhl, using Argentine 1909 parts scrounged from the DWM factory. The double S mark indicates chambering and fitting of the barrel by the Schilling factory. Valentin Christoph Schilling founded the company in 1816. V.C.S. closed doors in 1915. This dates the rifle 1912 – 1915.
The rust resistant Poldi Anticorro steel was first produced in 1910.
The serial number is Schilling’s own, not related to the Mauser numbers. There is nothing on this rifle that indicates any Mauser, Oberndorf, factory origin of rifle parts.