Here is a reference on Russian proofmarks; (You can navigate around the list on the left to find proofmarks from other countries.)
http://www.shotguns.se/html/russia.htmlIt looks like my TULA made "Baikal". In 12 ga. there would be a TO3 66 marking around the top of each barrel at the breach. I had a 20 gauge once and I seem to recall it was marked TO3 20. (The Russian "Z" looks like a "3", so pronounced TOZ 66)
If I am correct, these strong guns with tough springs are post WWII. They were not uncommon in Canada from the '60s, but I know they were later sold in the US, mostly 20" bbls for the Cas crowd (That's US!), and generally in 12ga. Mine (28 in. bbls.) has very good regulation, yeilding trap scores equal to those shot with more conventional field guns, but I have seen one that shot a foot higher with one barrel over the other.
You may have found a very shootable gun for our sport, and game at short ranges. If you are interested in the 16 gauge, look up <16ga.com>