The AJS 31 and its sister, the Matchless G12, were produced from 1958 until 1966. I don't think many came to the US after 1962 when the G15 and 33 came out - which where essentially the same bike with the engine enlarged to 750cc.
Only a little over 300 of the 750s were produced with the AMC engine - it was a disaster. The company ended up installing Norton Atlas engines in that chassis from late 1963 until 1968 (with a few rare exceptions), and built about 5000 of these 'hybrids', sold as AJS 33 and 33CSR, Matchless G15mkII, G15CSR and G15CS, and Norton N15CS and Atlas 750SS(Matchless CSR with Norton badging).
That same chassis was used in the Matchless G80 and AJS 18 bikes built at this time, too.
'73 Norton 850 Commando - 3030xx
'69 Norton Ranger 750 - P11/1289xx
'67 Matchless G15CS - G15CS/1235xx
'61 Matchless G80CS - 61/G80CS/41xx