I'm at work, and have my catalogs at home. I don't know who is still in that business. Danley's was, once, and Wentling after them. Every decade or so, someone inherits the stock of Kuharsky mounts (I think they made all the ones that B&L sold). The last one may have also bought & sold used mounts. I recollect that a 722 takes the same mounts as the 700, but I don't know about the 721. It should be pretty clear from the catalog, which I have at home.
Should be able to give you a detailed answer tomorrow, if no-one beats me to it.
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NRA Life member
This might be the gentleman that Bear Claw referred to. His speciality is old sights:
Gary Fellers
3249 Marie Lane
Ft. Worth, TX 76123-2055
Tel: 817 346-9633
Fax: 817 346-0690
In my quick glance at the catalog, the 721 was listed with the long-action 700's, the 722 with the short-action.
I was wrong about Wentling's. Apparently H.E. Gibbs bought out or took over Danley's, and as of some years ago, was at 23 Dartmouth St., Amsterdam NY 12010. His Phone number then was (518) 842-0688, but AT&T has been changing area codes right & left, so you might want to check the front of a phone book first, or do a search in on the internet.
The plunger-type mount is a lot handier than and just as secure as the leaf-spring mount, but I seem to recall that it can only be used with one-piece bases. Don't know whether anyone will have those for the 721. As a matter of fact, last time I dealt with Gibbs, they didn't have any plunger mounts, anyway, at least not new.
If you run into trouble getting info from Gibbs (like, he's out of business or something), let me know and I can send you some Xeroxes of catalog info. I even have a semi-extra spring mount, I think. The real issue, though, is getting the bases for your rifle.
Good luck!