As the original 1908 proofmarks also show a ledger number, 344, we know now the drilling was proofed and made in Zella-Mehlis, so the Suhl makers are out.
From the countless Zella-Mehlis gunmakers, Max Hermsdorf is not my favourite suspect. He was listed as a dealer in pistols and hunting equipment. His adress is the same as the "Adler-Waffenwerke", specialized pistolmakers.
As the stamp may be read as either "MH" or "TH" , I would look around among the countless Hengelhaupt's and Helfricht's active around WW1 in the Zella-Mehlis guntrade. If we read TH, it may stand for Traugott Helfricht at Forstgasse 9, who calls his business a "Gewehrfabrik" = gun factory, or Tuiskon Helfricht, Büchsenmacher. If you like to read an M, the possibilities are countless, as an M may stand either for a first name or simply for "Mehlis". Before WW1 Zella and Mehlis were still seperate towns.