Sending unit lock ring won't lock

I'm trying to get the lock ring back on the sending unit on my 71 Duster and when I put it on with the gasket, I can turn the lock ring by hand as far as it'll go and it never tightens. It's like the gasket is way too thin but the gasket I have is actually slightly thicker than the one that came off of it. I'm replacing the lock ring and gasket because it was leaking gas with the old one.

Anyone ever run across this before?