No, they are 10" You can get reproduction JBW ones that are 12x5's though. What has really put me of is that I would have to change my whole brake set-up. I'm going to carry on looking for 12x6's.
Old Dunlops, Cosmics and Mambas were classic 60s and 70s running gear for Minis. They fitted over 10" Cooper disks which cost a fortune nowadays if you can find them.