Noel,
You are right in one thing: the FG is an exception. It has the max aperture post reader on the mount, but it doesn't seem to be necessary.
The max aperture post was used on the EM (and the FG-20) for the auto flash functions, but on the FG it doesn't seem to work. The manual for the SB-19 refers that only the EM and the FG-20 can be used in Auto Aperture setting which uses the max post in combination with the AI tab to send info about the aperture in use to the flash.
As the FG has TTL flash metering, Nikon seems to have removed the EM flash system from the FG! Still, if you look at the mount on a FG, the max post lever is there at 5 o'clock position. And it isn't either necessary for the Program mode as the FG/FA use a close-loop system where they meter after the lens closes to the taking aperture, eliminating this way any errors due to sticky aperture blades or otherwise not the exact aperture setting. Program (and Shutter priority on the FA) mode is possible even with AI'd lens.
All AI modified lenses that I ever saw don't change the mount, only the aperture ring. So, the max aperture post isn't added and that's why the manual for the FA and the F4 refer that Matrix metering isn't possible with AI'd lenses as those cameras require it for the Matrix meter.
The Series E 35mm F2.5 isn't that rare. Ebay UK has 5 at the moment on sale, albeit at high prices. It is rare in the 2nd version with the chrome ring. My version is the 1st one all black and it is in mint condition.