There have been lots of discussion about optional Norton parts, for example dual seats because they are listed in the 1951/2/3 parts lists. This example is based on the singles parts lists.
The Norton part numbers from 1946 (post-war) worked like this:
A2/540 was the saddle for model 1, 16h, 18 and ES2 Nortons from 1946 to 1952.
A = the year = 1946
2 = the model number #2 = 16H
540 was the part number for the saddle
This part was first introduced in 1946 on a 16H. All saddles should be part 540.
Part A11/540 , a saddle, was introduced in 1946 on model #11 (International 30) and fitted to models 30, 30M, 40, 40M. The parts list shows:
A2/541 was the saddle spring for models 1, 16H, 18, ES2
A11/541 was the saddle spring for models 30, 30M, 40, 40M
So saddles for internationals and manxs where not the same as for the cheaper bikes.
In 1950 the parts list shows the change:
A2/540 saddle for models 1, 16H, 18, ES2
E2/541 saddle spring for models 1, 16H, 18, ES2
This change for the spring occurred in year E = 1950 on a 16H. The saddle spring changed from a horizontal bottom mount to a vertical bottom mount. The saddle diagram and related reference numbering in the 1950+ books is incorrect because it has been copied from the earlier manual and not corrected.
In the supplementary list for 1951/2/3 the dual seat was introduced in 1953 on model #12 (Twin Model 7)
H12/540 dual seat for models 1, 16H, 18 ES2
so the dual seat on the 1953 and 1954 models fitted single and twins with brazed frames.
I hope this helps.