The 500 cc "long stroke" AJS 18CS was produced from 1951 through 1955. In 1956 it was replaced by the shorter stroke, larger bore models that used the same numeric codes. The bore of these early models was 82.5 mm (3.25 in), while the 1956 through 1966 "short stroke" (final version) models had a bore of 86 mm (3.39 in). The C was for Competition (Scrambles) and the S for Suspension (not a rigid rear frame).
Engine numbers usually start with the year of production, followed by model designation, and completed with the production number of the motorcycle
The AJS and Matchless singles were doomed when AMC merged with Norton as all production was transferred to twins cylinder bikes - so the short-stroke Model 18 is the end of an era.
Pictures via : www.vintagebike.co.uk