Here are some more views of my Schlegelmilch Mauser, the shop your G&H barreled rifle came out of was likely identified on the barrel as this one was. It also may have had an octagon brl maybe accounting for what looks like gaps in the inletting from here(?) You might see traces of that in the brl channel?
(BTW, this is the styling I will be following for the custom Krag I'll be working on next year.)