I'm afraid I don't know exactly but i found this:
http://www.retrobike.co.uk/forum/viewtopic.php?t=165368
He states the number 6 is for boxxers and a 205 rotor, I'm 99% sure he means 200mm rotor as back when non mono m4s were around rotor sizes all seemed to be 160, 180, 200 etc. I think...