8x57, sorry, but I did not look close enough at the photo oft he opened barrel breech ends. And, you did not show a pic of the “action belly” before. Your drilling was made in Zella-Mehlis to the Franz Jaeger, Suhl, 1904 patent DRP 176578, with one lug under each barrel. Besides making such guns himself, Jaeger licensed Simson, Suhl, to make these actions, hence the breech action is known as the “Simson-Jaeger-Verschluss (= lockup). Several Zella-Mehlis makers also made copies, like your drilling. In Liege, Belgium, gunmaker Paul Scholberg offered basically the same action, called “Record”. Nothing of the Jaeger – Simson – Zella-Mehlis – Scholberg licensing arrangements is known any more. While Scholberg used monoblocs, the German makers used chopper lump barrels. The chopper lump seam is visible on the Greener extension of your drilling. All these actions were locked up only by a Greener crossbolt through the rib extension. Scholberg often used a square crossbolt. This lockup by a crossbolt only proved quite sufficient, “shooting loose” no more often than conventional underbolting. Remember, most Merkel over-unders, except the highest-grade sidelocks, are locked by the crossbolt of the Kersten or “Double Greener” extensions only.