I know the following to be fact.
The leaf springs that are on the car, are THE ONES that was on the car when I bought it the first time in 1979.
I know this because the rear sagged a little, and without removing them, dad and I heated and put about a 10 degree
bend upward, in the rear six inches of the longest leaf on each side. That bend was still there until I straightened it this year.
LEAF SPRING STAMPING LINE 1 -- "C4ZA5556N" -- LINE 2 -- "1 E" --
MAIN CARRIER CASTING DATE 7A7 34 CIRCLE WITH DOT -- 1 3 5 7 9 -- WITH 7 AT THE DOT
PINION CARRIER CAST NUMBERS D20WD
COULD NOT FIND ANY NUMBERS ON THE INPUT YOKE, BUT I DIDN'T REMOVE IT.
BOTH BRAKE BACKING PLATES HAVE FOMOCO SCRIPT OVAL
ONE IS STAMPED BENDIX 317565R THE OTHER IS STAMPED 70867 AND KH
The axle housing is the tapered tube type with a near round (no irregular shapes) dome over the gears. Only 1" dia dimples, right and left on the dome.
I'm thinking that a junk-yard rear end was put in between 1966 and 1979.