Old radiator

I'd call it the filler neck.

 

Tried a rad shop? What about the dealer?

A rad shop could probably weld a new filler neck on if you brought them the top tank.

 

What’s your 20? I have a good rad shop close.

 

Try Roppel Industries, they make radiators for most manufacturers.

 

That's an old school expansion tank on a very old school radiator. That's when it took a real man to rebuild them.

The company is still in business; maybe writing or calling them, they can provide you with some info.

By the way, I doubt any rad shop will know what it is, try a very old established shop with old farts still working on radiators or who haven't tossed their parts manuals away.