Yes, this is a Mauser Oberndorf symbol or cartouche of some sort. I first identified the variation when I discover that Spandau production was wildly erratic starting in 1917. Trends work on serial ranges and RR acceptance were totally incompatible, the gaps in serial ranges were extremely wide and erratic and the fireproofs on many of them resembled Danzig's style. When I compared the RR patterns, they were perfect matches for the same ranges in Danzig's production, which led me to consider other traits typical of Danzig production and they matched up well also.
This still left a small number of rifles (Spandau/1917 marked) that didn't have either Spandau or Danzig traits, it took awhile but I was able to match up Mauser Oberndorf RR acceptance patterns to these remaining rifles. It still left the unanswered question of the odd marking Ryan refers too, so I went through Jon Speed's books and found these pictures (attached- my association of the rifles to MO is because of the RR, this marking is just another identifier that they have in common). I assume it is some Mauser identifier, which is very typical of the Imperial era, you will find some "E" marked receivers (different host rifles) and they have Erfurt traits, the 1915 builds of older receivers also have the assemblers identifier across the top (CGH, S&S Suhl, Danzig etc..).
Anyway, I do not know the exact meaning or purpose of the cartouche or trade mark across the top, but I suspect it has to do with Mauser Oberndorf identifying their assembly. Usually we have the RR to prove this, about 7-8 are known, but in this rifles case I think it might have been re-numbered when re-barreled. Hard to say as the pictures aren't clear enough (detailed) and he doesn't show the RR fully.