I have always been under the impression that Clarkson made Autolock chucks in three sizes, small, medium and large.
I cannot remember the size range but someone might have a Clarkson brochure somewhere.
You should be able to find a medium in 3MT, but are unlikely to find a large. The medium ones are quite rare.
If you do find one, check it for runout as the medium ones can be too much cutter for the size of the shank. I had an R8 medium one that had suffered this. I gave it someone and he did manage to straighten it.