Neck sizes vastly differ from guitar to guitar on VINTAGE mustangs.
I've seen necks marked A or B for Mustangs, may have even been C and D necks for all I know. Kurt likely favored the A necks because they are the skinniest (and as it just so happens that's my favorite too).
B necks are similar to the average for most VINTAGE fender guitars. Meaning Vintage, made in the USA fender guitars, not Japanese made instruments.
The 70's Fenders might not vary so mucn, however, it was written, I think in Kurt's Equiptment that he had a 70's Mustang and did not like it because the neck did not feel right, which is understandable.
60's Fender Guitars tend to have more options (more colors, neck options, scale length options). When CBS bought them out, due to it now being run by a large corporation, the options were narrowed down to what would sell the most in the 70's. That's why almost everything in the 70's was either sunburst, white, black, or that wine red transparent color, because that's what sold, along with the black-out look added later. So Fender probably just picked a profile and a backshape and stuck to using just that on Mustangs, and not allowing customers the option to change this or that. While most Mustangs were not built to order, I'm sure there have been a few in the 60's at least.
Fender Japan is a whole different shebang...
Mustangs stayed quite popular in Japan in the 80's. A band called The Plastics used one for example. Enough so Fender made a USA run in 1984 specifically for the Japanese market, I really doubt these had any options as they were all pretty limited in color and style.
The current Japanese Mustangs came about sometime in the late 80's/early 90's in Japan, and probably started getting shipped over here far more once Kurt Cobain got popular and said in a few places that his favorite guitar was the Mustang. I have not played many of these guitars, but all of the MIJ and CIJ Mustangs I have played felt and looked like they had a neck identical in shape and design to my Jag-Stang.
I have this thought that on the Japanese guitars, Fender may have just had ONE 24"scale neck being made at first.
Forwards Headstock - Fender Jaguar, Fender Jaguar HH, Fender Mustang, Fender Jag-Stang, Squier Bronco, Squier Vista Jagmaster, Left handed Squier Super-Sonic (if such a thing exists)
Backwards Headstock - All lefties of the above, and the Squier Vista Series Super-Sonic
The reason I think this is because I have played almost all of the above save for left handed variants and the Squier Bronco/Musicmaster guitar that was short lived, and all of them were somewhere in the same ballpark as my Jag-Stang and Jaguar, which are MIJ and CIJ respectively. The Vistas felt closer to my Jag-Stang, and my Jaguar feels slightly chunkier. I also think the Jazzmaster used a unique neck in that all the MIJ Jazzmasters I have played felt like my Jag-Stang in neck shape as well, and were very thin.
