Help identifying my LS6!

[VIIX]

I just registered with your forum because I am a huge Yenko fan and also need help identifying an engine block that I just purchased and am rebuilding. I purchased a complete LS6 engine (minus the carb) from a customer that I have known for over 20 years who is a fanatic about originality and authenticity with his old Chevys. He has had the engine for well over 20 years and he knew the fella very well that he bought the engine off of. As my customer has gotten older he hasn't been able to remember the exact details of the origin of this engine - only that the guy he got it from last had it in a street rod in the 80's. My dilemma is that the date code for the 512 block is D 7 71 the 291 heads are L 2 70 and B 5 71. The suffix on the engine is CRR with other numerical stampings that I did not write down. In all of the publications that I read, the CRR is only designated with a '70 Chevelle with an automatic. I have been told by guys that were around in the 70's (I was born in '76) and worked in the dealerships that GM never stamped an over the counter LS6. Any help and direction would be greatly appreciated.
Back in 1995 there was an ad in Hemmings for a real 69 Yenko Chevelle that need to be restored, but complete. I talked to the guy and he would have sold it for $15k. I just couldn't seem to justify the purchase being so young at the time. If I could only go back!