The Kreighoff-Patentmagazin (stock magazine) you describe was a factory feature offered by the H. Krieghoff company after WW II, and for all I know may still be available. The post-War version involved a detachable buttplate, however, rather than a trap like the one installed in your drilling.

The purpose of the auxiliary rear sight is a little unclear, since the insert barrels were usually adjusted to a point of impact identical with that of the larger rifle caliber, so that the scope could be used with both the regular and insert barrel.

You are undoubtedly correct that the scope mount was a hand fitted arrangement and would therefore the bases would not be interchangeable from one scope to another. Perhaps some hand fitting might correct the situation, unless the spacing of the rings makes this impossible.

I trust you are aware of the need for using properly sized (.318" diameter) bullets in your 8X57 JR rifle barrel, rather than the more common .323" bullets.

You have a fine drilling, in every respect that I can see identical with the post war offering of the Krieghoff company, even to the engraving. Congratulations, and welcome to the forum!