Hitch receiver's a weird size

This could be more age related than van related but since it's on the van I'll post it here.

I had a hitch receiver on the 1989 roadtrek with a square hitch insert fused in from rust. I soaked it for days and hammered away on it with a ball peen hammer and finally used a sledge hammer to knock it out.

Then I bought a bike rack and the hole was too small.

It's as if the part on the bike rack is exactly the same size as the receiver. If I had a coupling it would likely fit inside both.

The outside of the bike rack piece is 2" exactly and is intended to fit inside the receiver. But the receiver measures about 1 3/4" inside.

I'm puzzled. Was there a time when there were alternate sizes or does it need some sort of a coupler?

By the way, the bike carrier came with a 1 1/4" adapter and it's far too small so I don't think that's the issue.

thanks as always for the ideas.