I have been all over it. Even put a micrometer on it to make sure it hadn't been filed off and then sanded down smooth.
Other than not having a stamp, here is why I always believed it to be a service replacement housing.
The original round pinion snubber has a chunk missing. When I pulled the gas tank, and found the date code, it was dated Jan of 1970.
So, I always assumed there was a catastrophic failure that took a chunk from the snubber and knocked a hole in the tank.
I don't believe the car has ever been out of Oklahoma. Shipper report confirms it was sold here, but no one has figured out which dealer. Normally, an Oklahoma car can be traced back to day 1. But, as luck would have it, mine is one of the few that cannot be traced back. The Tax Commission (the agency which handles titles in OK,... think of the DMV in most states) lost large portions of information when they changed computer systems in the late 70's. They don't even have a record of my original purchase in 1976, nor my brother's in 1975.
Attached a clearer shot of the date, as well as a shot of the axle tube.