12 bolt axle ID help

[d3mon]

The axle does not appear to be original as the shock mounts are fabbed from angle iron and there are lines across the top of the axle where it looks like a pair of spring perches were located and have since been moved outward. The arrows show these grooves. There is a yellow grease pen writing of 3.20? on the top and the gear ratio comes out in this ballpark as well, as if it came from a salvage yard. There are no stamping numbers that I could find on the tubes. Below is all the information I know.

Off the differential housing:
C0N20
123456
67 90
E24 20
3896839

57" from backing plate to backing plate, 6 lug axles and is a limited slip.

The vehicle I am building is a 69 GMC Fleetside SWB

I am just trying to figure out what this axle came out of or why they had to relocate the shock mounts and spring perches.

Thanks in advance!
Ryan