an early 18MS It has 'candlestick' rear suspension units that were used in 1949 and 1050. It may not necessarily be an 18 - it could also be a 350cc Model 16. I recognise candlestick (have still 2 pairs as spare!and also a lot of jampot) it's what I say (wrote) "early" barrel looks big I ride same bike, a'55 16MS in '57!
My first motorcycle was a 1952 AJS 16MS - that was in 1969.