From the marks it appears it(maybe just the action) was proofed in Suhl prior to the change in the proof rules in 1912 and then again about that time or a little later before the September 1923 dating scheme, but I don't see a Crown over R so there may have been some rebarrel effort post 1912. I see the pre-rifled diameter in mm so that couples well with the Nitro proof post 1912. Not sure on the script E 3 B, maybe a EB the 3rd from a 3rd generation of mechanics. But from the effort on the tube, I'd say Robert Schlegelmilch is the holder of the encirced RS. Are there any pre-1910 plug gauge diameter type stamps?

Kind Regards,

Raimey
rse