looked at the rear out of my 66 SS today and it reads 3917123NF with an X above 71, GM57, and 67890 or 67880 can't tell the forth number. I know this is a 10 bolt but out of what. :confused:

You need two codes. First the cast date which includes the year. Look at the rear center section facing the cover and at the 5 or 7 o'clock position next to the opening, will be a small raised letter for month and numbers for day and last digit of year.(B 15 6 is Feb 15th '66 for example)
The axle code,assembly date, axle plant, is hand stamped into the "forward" side of the passenger axle tube. It will look something like
CA 0215 B for example. Very shallow stamping so needs to be cleaned down to bare metal to see.
This is the 66 list:
http://www.chevelles.com/years/66/66rearcodes.htm

Rene
66_Malibu

I think it is a 1968 Chevelle 10 bolt. 3917123 is one digit less than 3917124, which is a 1968 Chevelle 12 bolt. This is the pattern followed by other model year Chevelle 10 bolts and 12 bolts.

I found the numbers stampted into the rear it is CB 0529B that means 3:36 open rear, may 2 1969 made in Buffalo plant. right?

May 29th 1968.

What was the small raised casting date on the center section?
That verifies year to be sure.