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All rings for the CZ 550 work. The problem is that they are all pretty low and you won't be able to mount a scope larger than 40mm objective as the bell is going to touch the rear sight. If you like to setup a Weaver K4 style scope (which is has a nice look on a 21H as you can see) you can go with any HIGH CZ 550 ring made by CZ, Warne etc.
I have a set of like new CZ rings that fit a steel Weaver or small Leupold. PM me if you are interested.
Higher rings for larger scopes (50mm +) are only available in Europe. I can hook you up with a company that manufactures higher mounts.
My 21H sports now a 3-9x50 Nikon Buckmaster on high look-thru rings (I can use iron sights). Little weired look for NA eyes but closer to the traditional European setup which was always very high. Nice shooting rig!