It appears the Cirkus was added starting in 1993 with serial number 90,582.
The Cirkus bearing is flanged out at a 45 degree angle as it mates to the bottom of the subchassis. The original bearing is not. I think I have a picture of both kicking around somewhere. The color of the bearing well is not relevant. You might also want to check the pink fish media site, which, being a UK based site, has a tremendous amount of insight to offer regarding the LP12 (as well as the pros and cons of the pre/post Cirkus sound).