That's the same barrel shape like the one shown by Xausa. Note the step in front of the small reinforce. The sleeve of the rear sight abuts against this step. BTW, the tangent rear sight blades of the commercial rifles are smaller than the military ones. In fact they are the same size and interchangeable with the ones used by Mauser on their C96 "broomhandle" pistols and their smallbore .22 lr rifles. Just the graduations differ.
The photos of your barrel channel show black streaks, the traces of a former octagonal barrel. Apparently the inleting has been altered before. Perhaps the rifle had been rebarreled before to a barrel of different profile?
As it is near impossible to reproduce a half-octagonal, ribbed Barrel of the Mauser factory style now, you have to compromise in some way.
Either you use a round barrel of a correct Mauser style, E type or the one shown by Xausa, and alter the barrel channel accordingly by inletting and glass bedding,
Or you use a non-Mauser barrel profile that fits your barrel channel as it is now,and live with it.