The differences between the 18 and ES2 varied according to the year but the ES2 was always the more expensive, more fully specified model (not that any Nortons were ever Bargain-Basement).
If you are thinking about the 1932-on Edgar Franks redesigned models, the basic difference was that the ES2 had the full cradle frame and could be specified with larger sports fuel tank etc. The 18 shared its open-diamond frame with the 19 and the side-valve models. Plunger rear suspension was an ES2 option for 1938 / 39 but as I recall from the factory records, not all that many were made.
Post-war production was standardised upon a cradle frame in 1946 and when the ES2 was re-introduced in 1947, it had plunger suspension as standard and the whole range gained Roadholder forks. Engine units are identical except that the model code was stamped next to the number (3 for the Model 18 and 4 for the ES2).
Model 55 production is difficult to quantify and would require a page by page search of the factory ledgers. The 350s had their own series of numbers but this was for Model 50, Model 55 (the twinport version) and also the CJs and Internationals. The twinports are not common but could have received single port Model 50 heads in the intervening years.
My guess from the records would be that 350 production was in the low thousands per year and twin port production a smallish percentage of that figure. This would suggest perhaps 3000 or so made but this is just my impression, it could be wrong !
The Norton Owners Club have been compiling a database which will enable this sort of information to be extracted.
The Model 50 and 55s did have different frames and crankcases etc. from the 500/600 range (1,16H,18,19 & 20) pre-war so interchangeability is not high even though the parts look the same !
