The rifle originally with your barrel was retailed by Alfred Plett, Neue Mainzer Str. 77, Frankfurt am Main, but he did not make barrel or rifle. The original rifle was made for him by the shop of Christoph Friedrich Triebel, Grosse Backstr. 14, Suhl, a well known gunmaker to the trade. Triebel used a barrel made by Wilhelm Kelber, Beiersgrund 3, Suhl. As the two proofdates and the crown/R = repair proofmark show, it was altered, rechambered?, soon. Maybe rechambered from 8x60 to 8x64 Brenneke? An 8x60 cartridge would also slip into an 8x64 chamber and seem to fit. So a chamber cast is necessary before you buy ammo or reloading dies. 8x64 brass is easy to make: Neck up 7x64 cases.Load data is availble for both cartridges or you may use 8mm-06 data for the 8x64.