Thanks for the clarification.
RE6B is the heat lot. The KK is the inspector's code.
So far I have seen RE6, RE6A, and RE6B. If yours had been RE6R I would not have been surprised, but more than likely it would be a mis-stamp, an anomaly. I have seen this before. There is a rifle I've seen with heat lot BE4A and I'm sure it is really a RE4A.