This AJS model 31 is equivalent to the Matchless G12, which means it has a 650cc engine. As best I can tell, it was manufactured from 1958 to 1966. The standard models had a 7.5:1 compression ratio while the CS/CSR's had 8.5:1.
The optional colour scheme on the 1961 was reversed...Arctic White on top, blue below on gas tank. The bike was very original but completely dissasembled when last acquired. At that time, the frame was painted metalflake green and many painted parts had been chromed....it was going to be a "Chopper", before being salvaged and restored. The paint colours were hand mixed closely matching the original.