it's a bit hard to tell without seeing any of the mating surfaces or being given any dimensions, but it looks like the Universal sub-mount you find on a lot of Cooke and Angenieux zooms and certain telephoto primes. The thread is 58mm diameter, a pin locks in the orientation.
Commonly adapts to PL, but I've seen it in other mounts like Arri S and B, CP, Aaton, B4 etc. Duclos makes one for Canon EF mount. Here's a PL version from Visual Products:
I'm curious, did you ever find out the cause of your soft footage? If the lock ring on this sort of mount comes a bit loose the back-focus will be out. It's also possible to misalign the pin so that the mount is not seated properly.
Yes! That must be what it is. It has the locking pin and everything. This information is tremendously helpful. Because let's face it, a lens that's only good for the Eclair is of limited utility...but a lens that can be adapted to a host of mounts including PL...that makes selling it a much better proposition.
Now as for the backfocus issue...I couldn't replicate the cause. I tested the lens at a half dozen different distances, and every time, it measured out perfectly. It was collimated by one of the best techs in the biz, Bernie O'Doherty. We talked it over too, and he felt confident it wasn't the collimation.
So I feel what happened was I didn't have the Eclair turret locked down. The Eclair is rather notorious for that. It was a cold day, I had gloves on, and I was in a hurry to film a the event, which was the annual Christmas lighting ceremony. I think I simply messed up and didn't lock the turret, which as we all know, if it's off by just a mm or two, will blow the backfocus.