Help Understanding hitch ratings with a bumper and ball.

[SS Tim]

In your case the bumper itself between the hitch ball and your "grade 8" bolts is the problem and the unknown as far as a posted capacity.
It is not at all uncommon to see these on trucks that are used and abused to have the center all tweaked and veed out by some ill advised load.

Here is a load chart for '72 from GM. Remeber these are the truck's rating with the correct frame mounted hitch which a step bumper generally is not.

Grew up using U-Hauls and probaly have a couple of their 2 1/8' balls still, maybe even a 2 piece relic.