I have had several of these and several ZG-47s, still have 3 21/22s, 3 ZGs and one superb action of each. They, are not hard to find here in Canada and are among the finest actions ever made.
The roundtops are the early version and the integral scope bases are the later ones. The best of all are the last ones with the smallring, integral bases and ZG bolts with locking safety.
I do have one original roundbolt, one conversion by Martini and have never even seen a 21-ZG type in 50+ years of dealing with these.
For those who think a P-64-70 is the pinnacle of bolt hunting rifles, see one of these and see what IS the finest.
I compared the Brno 21 H 7-57 to one of the old M70 Featherweights I have and the smaller Brno is a handy rifle however I am used to the Winchesters. The stock on the M70 is somewhat 'thick' after you handle the slender M21.
The safety on the M70 is far superior. That's why I had a Dakota put on the M21.
The walnut on the M70 is of a high grade with great figure. The 21's is not.
That M70 of mine weighs 7#11oz with it's 4X Bear Cub in Buehler 2 pc mounts. The 21H goes 7# 9 oz in the original factory mounts.