1.4 TC use for the R5 and R6

Northroad wrote in post #19273006
So I tried stacking an EF 2X III and an EF 1.4X III with an EF 12 II extension tube between them. The front of the 1.4 was extremely tight going into the rear of the extension tube, I had to force it. When I disconnected, a plastic piece came out. Now they go together and work just fine and I'm wondering if this little piece of plastic with a too small hole in the middle is even necessary.

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Looks like it might be there as a baffle, to keep stray light from passing through that is not productive for imaging (flare).

Northroad wrote in post #19273006
So I tried stacking an EF 2X III and an EF 1.4X III with an EF 12 II extension tube between them. The front of the 1.4 was extremely tight going into the rear of the extension tube, I had to force it. When I disconnected, a plastic piece came out. Now they go together and work just fine and I'm wondering if this little piece of plastic with a too small hole in the middle is even necessary.

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Have a look at the protruding front element of the TC. It has a rubber protector. I think your's must be either damaged, or bunched up.
I never noticed it to be a tight fit, I just tried my 1.4TCIII in the EF 12 II and there is significant space between the rubber protector and that baffle.

I measure the baffle as 34.5 mm diameter. I measure the TC as 33.1mm.

The rubber protectors on both of my TCs are slightly larger than the hole. I can squeeze them in but have to fight to get them to come back out. I wonder if I should work that hole over with a file or a Scotchbrite pad to gain some clearance.

Bizarre. The 1.4mm clearance I measure sounds small but there is a lot of movement when I place the tube over the TC.
My tube definitely has the baffle. I suppose it is probably not very common for people to fit the tube over the TC, though of all those on POTN that have, nobody has mentioned the tight fit.

It seems like either the wrong parts bin was used for your TC or Canon changed suppliers for the rubber protectors. My 1.4TCIII is quite old, would have been one of the early production runs.

Northroad wrote in post #19273834
The rubber protectors on both of my TCs are slightly larger than the hole. I can squeeze them in but have to fight to get them to come back out. I wonder if I should work that hole over with a file or a Scotchbrite pad to gain some clearance.

When it comes down to modding a relatively cheap extension tube or an expensive TC, I know what the answer would be.

Used a piece of Scotchbrite to wallow out that hole. Took a few tries but now it fits like a glove.

I just use the kenko 12mm with my 1.4 and 2x, it all mounts just fine. AF is a bit slow though on the R6 as would be expected.