Thank you gentlemen for the replies. I looked through the introduction material and the model photos and didn't see an associated A, B, C designation; didn't see a model differences page proper. I haven't bounced the part numbers against the body changes you mentioned, but will shortly.
I did learn my chassis number puts the date of manufacture between September '65 and March '66 - more info than I had before! I don't suppose Nissan does "birth certificates" like VW...
Again, thanks for the input; much appreciated.