Here are some facts after a few tests.
The Canon EF 2x III teleconverter works perfectly with a Canon EF 400mm f/2.8L IS USM.
With the Canon 100-400mm EF f/4.5-5.6 L IS II USM as well as the 4.5-5.6 L IS USM it doesn't look so optimal... Although it is on the compatibility list.
It fits mechanically, the aperture settings also work, but the autofocus does not work!
In manual focus mode, the pictures with the 100-400mm EF f/4.5-5.6 L IS II are quite good, which I can't say for the older model (100-400mm EF f/4.5-5.6 L IS).
The difference between the two is clearly visible even without the extender.

Here are some facts after some testing.
The Canon EF 2x III teleconverter works perfectly with a Canon EF 400 mm f / 2.8L IS USM.
With the Canon 100-400mm EF f / 4.5-5.6 L IS II USM as well as the 4.5-5.6 L IS USM, it doesn't look so good... Although it is on the compatibility list.
Mechanically it fits perfectly, the aperture settings also work, but the autofocus doesn't work!
In manual focus mode, the images with the 100-400mm EF f/4.5-5.6 L IS II are pretty good, which I can't say with the older model (100-400mm EF f/4.5-5.6 L IS).
But the difference between the two is clearly visible even without extension.

Sorry for the question who seeks finds .... https://store.canon.ch/fr...

EF 600mm f/4L IS USMEF 400mm f/2.8L IS III USMEF 600mm f/4L IS III USMEF 800mm f/5.6L IS USMEF 500mm f/4L IS II USMEF 500mm f/4L IS USMEF 400mm f/5.6L USMEF 400mm f/4 DO IS II USMEF 400mm f/4 DO IS USMEF 600mm f/4L IS II USMEF 135mm f/2L USMEF 300mm f/2.8L IS II USMEF 300mm f/4L IS USMEF 200mm f/2L IS USMEF 200mm f/2.8L II USMEF 300mm f/2.8L IS USMEF 400mm f/2.8L IS USMEF 400mm f/2.8L IS II USMEF 180mm f/3.5L Macro USMEF 28-300mm f/3.5-5.6L IS USMEF 70-200mm f/2.8L IS II USMEF 70-200mm f/2.8L IS III USMEF 70-200mm f/2.8L USMEF 70-200mm f/4L IS USMEF 70-200mm f/4L IS II USMEF 70-200mm f/4L USMEF 100-400mm f/4.5 - 5.6L IS II USMEF 100-400mm f/4.5 - 5.6L IS USMEF 200-400mm f/4L IS USM Extender 1.4x