the exposed nitro proof on the lower portion of the shotgun barrel chambers, was begun in april of 1912 at the suhl proofhouse, and from memory continued to be stamped in that area until sometime in early 1920's - when it was moved to the barrel flats, and the three or four digit dating stamp commenced. after looking at your photos again, i notice in the final picture what may be 8/24 stamped on the left side of the rifle barrel - which may be august 1924, but i can't clearly see the stamp. the mark "St.m.G. over 15gr." indicates the rifle proof was done with a steel jacketed bullet of 15 grains weight.
reality indicates that likely production during the great war would be significantly diminished, more so in the latter years of the war, so there would a quite small window before the war....and with sporting gun production increasing following the armistice.
best regards,
tom
