Receiver proofs are 280 X three on the right like ERMA ???
Yesterday my answer was quite short and now I will go a bit deeper into the details.
In 1937 in the end of "e" block the Waffenamt inspection team "280" took over the inspection and acceptance at Mauser Berlin-Borsigwalde, that team stayed there until late 1938 in the late "o" block. In the early "p" block of 243 1938 you find the Waffenamt inspection 280 (x2) and 26 (x1) on the RR, in the late "p" block 280 (x1) and 26 (x2). In late 1938 the Waffenamt inspection team "26" took over the inspection and continued until the end of K98k production at Mauser Berlin.
During the time the Waffenamt inspection team "280" did their service at Mauser Berlin-Borsigwalde, the team "77" took over the inspection and acceptance at Erfurter Maschinenfabrik and after their service team "280" moved back to Erma.
Please show some more details of your rifle.