dont understand the xx (have you left out some numbers there) the serial numbers on mine are 06273 / 09053 / 13275 mk 1s and 49992 mk2 the mk 2s have mk2 stamped on the breech block and have a plainer (shaped) stock usually mk 1s have a plastic trigger blade but sometimes that gets changed and i have seen quite a few mk2s with what appears to be mk1 stocks .the calibre thing is another puzzle (why didnt they stamp it on the barrel)