In an email Ardent told me the barrel being marked "7.57" underneath. The "7.57" seems to be missing a digit or two and it shows the barrel is not of Mauser origin, but Austrian. You can bore out a barrel from 6.5mm to 7mm, but you cannot "bore in" it the other way. I suspect a "6," is missing or was removed while installing and finishing the barrel. "6,7.57" would be a common designation of a Ferlach barrelmaker. Remember, many European cartridges got their first number from the bore/land diameter, not the barrel/groove one. The bullets of many, but not all, "6.5 mm" cartridges are .264" = 6.7 mm actually, just as "7mm" .284" are 7.2 mm actually. Other than the Germans, the Austrians sometimes used the real bullet diameter in the cartridge designation. This leads to confusion sometimes. F.I. I have a package of old Austrian 6.5x54 Mannlicher-Schoenauer cartridges labeled and headstamped "6,7x54 M-S". So I think the 6.5x57 barrel now on the action is of Austrian, Ferlach?, make and was installed and proofed in 1942 on the behalf of R.Mahrholdt. The rifle was not merely rechambered as I thought before, but rebarreled.