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Sorry, just back from hols.
Yup, looks identical to the cervelo lockring I had. I borrowed a rotor lockring tool from danstuff and bought a new regular lockring. Removing the cervelo lockring was a PITA because it was super tight and I didn't have an appropriately sturdy bench/vice but I took it to my local engineering workshop (agricultural machinery specialists who were stood on top of a combine harvester beating it with sledgehammers when I arrived) and got it off with the help of a length of scaffolding.
New lockring hasn't done a lot of miles as it's on my tt bike but it now fits and seems to be fine after a few months of racing.
Have you got the weird cervelo specific cranks?