Your drilling was proofed at the Zella – Mehlis proofhouse and made by the Z-M guntrade, evidenced by „Nitro“ in script and the proofhouse ledger number below the proofdate. At least the barrel lining job was most likely done by WK, here one of several Z-M Willy Krausser’s, not the well known Suhl barrelmaker Wilhelm Kelber, who used a slightly different WK stamp. The Repair proof is dated 6/36, June 1936, gun number 288 of that month at the proofhouse. At the same time the shotgun chambers were lengthened from the formerly standard 65 mm = 2 ˝” to 70 mm = 2 ľ”, hence the “Repair” mark on the shot barrel too. 16/1 is the actual gauge number of the barrel bore, while circled 16 is the cartridge gauge. The proofhouse simplified things by merely stamping the “Repair” marks over the original “B” proofmark without restamping the other marks. The N / K.m.G. = Kupfermantelgeschoss / 12 gr = Nitro/ copper jacket bullet / 185 gr service load marking is a leftover from the former 9.3x72R proof. The new service load marking for a 11,5 g = 177 gr steel jacketed bullet is a bit enigmatic, as that weight is quite excessive for a 6.5x58R, that used 8.2 g = 126 gr .261” bullets usually. Perhaps someone at the proofhouse goofed, stamping the same charge on two guns or mixing up things? Shit happens!