Also, the 0723 is the Assembly Date (July 23rd).
If original to the car, it looks like you have a very late built '64.
By the way, this is the same type Rear even if it had a 409, although most 409's were ordered with a Posi Unit.
The Posi may have been mandatory with a 409 order, but I don't believe it was.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America