A3 scope mount

[ArtPahl]

I would suggest you try Weaver #59 (rear) and #45 (front). They may be a bit hard to find at this late date, but they were a popular mount system in the 60's when the DCM sold a lot of A3's. The #45 should be easy to find since it's used on Mauser rifles: The #59--not as easy! I'd try the #45 first to see if the hole spacing mates; if it doesn't, the #59 is moot.

I found a #45 base in my "stuff" and the hole spacing is exactly .500" and, from your measurements, it appears that's what you have.

Ebay has lots of 'em--both numbers. And, of course, rings for Weaver bases are everywhere.