buckstix,
Upon further review and re- consideration, I no longer believe your rifle has the lowest numbered transitional action known. In one of his comments, Axel mentions one numbered in the teens, whereas yours is 27.(Note, it is pretty hard to evaluate these rifles when they are posted under different threads in different forums and some photos open while others may not) By the way, If I tell you something and Axel tells you something different in the same or different forum, you should accept his comments as authoritative. Since I think the action was used in the testing and later sold to Sauer and had a civilian rifle built around it, which was still later donated to help the war effort against the British, it would have markings from different sources. Most of the markings come from its life as a civilian rifle and its production as one. The numbers 27on the action are from Mauser, except the one on the barrel next to the receiver ring, which was Sauer matching their new barrel to the correct receiver. The number 76838 is the Sauer serial number and wherever there is an 838 it is Sauer's procedure to match parts to the serial number 76838 . The number 172/28 is the proof mark, showing the bore (not groove or bullet) diameter as expressed in gauge measurement. The 2.75g G.B.P. over St.M.P. is the nitro proof load, meaning 2.75grams of rifle flake powder with a steel jacketed bullet. The two crowns, one of which is over an N is the proof mark showing nitro proof of rifles. The little men, affectionally known as Wildman are Sauer's in house quality marks and are not official proof marks. The Imperial eagle on the barrel flat was applied by the Imperial government to accept the rifle for use in the Great(large)War. The bore diameter being shown in gauge measurement means Sauer built it before the 1911 improvements to the 1891 proof law were implemented in 1912. It would have been built after testing in 1896/7 was completed; so, between 1898 and 1912. Unfortunately, the original Sauer stock seems to have been replaced with a more modern style one, and unless the person you bought the rifle from can produce it, the identification of the military unit it was issued to is lost. Other single numbers or other marks are likely workmen's touch marks and since there is no ledger of them the workmen cannot be positively identified. I hope this helps.
Mike