I have a few real 68060RC50 & RC60 rev6, marking down the left hand side has 002,004 & 006 ending, but I am only willing to sell a de-soldered 68060RC50 rev6.
I have to agree with the other posting here, why would you put a heatsink on a rev6?
FREESCALE marking, you can't go much wrong here & should be real.
One other note, you can overclock some older revisions to a higher than normal overclock frequency, but you have to Increase the voltage a litter higher than 3.3v. ...I forgotten which older revision this works on, but if I find time I will check & post again. If I remember correctly I think I raised it to 3.45v.