The story continues ... with the help of fellow members here we re-located a mount which was once not identified, but when comparing it to pictures of a known mount, we knew what it was.
The in-the-meantime buyer considered the high offer I made and it now finally arrived with me. No doubt it is the correct mount for this rifle, and it also gives the answer what the cheekpad was intended for. I need to admit that it doesn't fit that well (dovetail on the mount seems to be larger than on my rifle), but they might had been hand fitted or probably someone altered it after WWII, who knows. At least it locks in place and looks good the way it is now.
The WaA63 and the other marking on the mount seem to have been added later, at least I suppose it to. But I consider this to be a luxury problem. Note that even the top of the split rings were numbered/marked (even on both sides!) for the mount on the inside, plus the small lines showing the right position for the scope rings in the upper scope mount. The side adjustment is also possible, as with the Short Side Rail snipers, via a dovetailed rear ring.
I fitted the first spare scope from my display. The rings however look like they still have space for a thicker tube, and therefore I suppose the CAD or the H4/60, having a half-a-milimeter-thicker tube would be the correct one for this rifle. Good that I have a spare CAD which a friend is currently cleaning

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Looking forward to comments on the now re-completed rifle. And in the meantime I gotta go and get some lottery tickets while I'm on the lucky streak....