not so unusual, I have seen such combinations more than once but most times a 4x and a 8x scope. why? because there wasnt a variable scope in this time and the ziel 4 scope comes to an end on longer range. its true that such big scopes were very expensive in this days and out of the reach of the common hunter.
you are maybe right that the second base on the barrel was made later, maybe post war. good or bad that only the original scope survive with the rifle. whatever the new owner wish a most modern or period scope it can be done of course.

I would look for an old scope, imho