That is a very nice rifle.

The Crown over R indicates a reproofing due to a repair. I think the flats on the side of the receiver are from removing previous markings on the left side of the receiver and the right was done to balance the esthetics.

Perhaps it started life as an 8x57mm but due to laws after the Germans lost WWI military calibers where not allowed so the chamber of the original barrel was cut to a legal 8x60. Or, for some reason, the barrel was replaced with an 8x60mm which would explain the lack of any other marking on the barrel. With the chamber recut or the barrel replaced it would need reproofing and the old receiver proof mark removed from the side of the receiver and a new marking applied so that could explain the flat sides of the receiver.

I personally would not worry about the front scope base being dovetailed into the receiver ring. If it was good enough for Mauser to do on sporting rifles they built it should be okay on your rifle.

Vlad