I know this thread’s a few months old, but I love Ruger Marks in general and Mark II’s in particular, and I’ve tried almost every classic original and several current non-drill&tap options for mounting scopes and dot sights.
Here’s a dot mount for the Mark’s rear sight dovetail I’ve had great luck with, which came to $28.81 to my door, including shipping & tax:
https://www.reddotmate.com/product/dot-sights-mount-plate-mk-iv-22-45/
This outfit ships from China, but they have a wide range of dot mounts for many different guns (Rossi R92 anyone? Huben GK1, Beretta 1301, Taurus revolver, TX22, KelTec P17, etc.), and covering up to _nine_ different dot footprints. The guy was extremely responsive to my email questions and requests, and the mounts arrived in about 6-7 days, IIRC. This was my experience in Nov. 2025, YMMV.
I love to buy American, but at $40-$70 per base, plus shipping and tax often, you can be into the price of an entire pistol pretty quickly. I haven’t tried any of their mounts for reciprocating slides, but the Marks don’t reciprocate, so there’s no real stress on the mount, and mine were nicely machined and installed easily.
Here’s a shot of one installed for a basic Crimson Trace on a Volquartsen 4.5” match-barrelled Mark IV upper:
[Edit 2/11/26] - FWIW, Volquartsen still has these threaded Mark IV uppers on clearance for $225, free shipping, no tax, which is a steal:
https://volquartsen.com/inventory_configurations/8505?clearance=1
Anyway, I hope this helps.