I found a 66 chevelle 12 bolt rear end for sale. The date code on the housing checks out, f106= June 10 66 and the stampings on the axle tube is correct cw0621b = 3.31 non posi june 21 Buffalo. What is throwing me is the casting number of 3775745n. I can't find this number in any resource material. My research shows the number should be 3875745n. I thought maybe the seller misread the number but I looked at the rear end and it does have the 377 number rather than the 387 number. Any help in solving this mystery would be appreciated.

Welcome to TC.
Yeah , it should be 387 but could just be a foundry fluke.
If your not going for the perfect replacement ,just get it and go.
And FYI ,the 3775745 seems to be a front bumper brace for a Corvair per the parts catalog.

Thanks for the comments Mike. Assuming it was an error at the foundry, does that really impair the value of the rear end as long as the dates are correct? Could anyone really question that is a true 66 Chevelle rear end?