Looking at the image of the original converter, I realized it looks very similar to a Canon 1.4 teleconverter, which also has this protruding element and can be used with only some larger teles:
In order to get something that connects to the EOS mount, I bought a (cheap) M42-EOS adaptor. The adaptor is not totally flat on the front, so it was flattened first
After removing the back of the 1.4 extender, I measured how thick the "ring" that goes between the M42 and the converter front needs to be and finally settled on the magic value of 20.4 mm between the EOS and the FD mount (20.4 mm includes the front part of the 1.4FD and the M42)
The "ring" was made by a friend and connects to the 1.4 FD through the original screw holes. To attach the M42 3 new holes were drilled into the EOS flange of the M42. The little "stopper block" was added to push the "stop down lever" on the FD lens and therefore to allow to stop down the lens which would otherwise be fully open all the time. When mounting the lens, one has to rotate it slightly into the other direction to engage the stop down lever first, but this is easy to do and works fine. (UPDATE: the stopper block was a bit to short and therefore with some lenses the stop down lever "slipped" past it, it is now glued to the back of the 1.4 FD in the same position - works fine and still can be easily removed if needed)
I wanted to leave the 1.4 FD unmodified, but the "tube" on the back side goes too far into the EOS mount and blocks the mirror on both my EOS600 and EOS33, so about 1.5 mm were taken of:
After assembly the new 1.4 FD-EOS converter is ready to be used:
Now put on FD and EOS caps :-)
EOS 30/Elan 7E with FD 300/2.8 attached:
