Which 2x extender for the FD 500mm Reflex?

On their FD range Canon, concerning 2x extenders, advises their type A extender for f.l. of 300mm and more, their type B for less than 300mm.
This seperation seems to be taken over by all authors. However, looking at other lens programmes concerning this issue I found one range where the type of lens was of more importance than the focal length.
And to my understanding the FD 500mm R, as its back lens element is nearer to the camera than with the standard tele-construction, has a wider exit angle than the standard type. Thus with reference to an extender it should behave more as a shorter lens. Making it apt for the B-type extender?

Has anyone tested both FD2x extenders on that reflex lens?

I assume I'll have to acquire both extenders and perform tests by myself.

If you're passing through New York any time soon, drop me a note and we'll test them. I have them both.