Technical MODEL A FUEL CAP OR RADIATOR

I'm looking for both a fuel cap and a radiator cap but I can't tell the difference. It's for a 29 sportster and they both look the same on ebay. I don't want to but two of the same. Thanks for your help

 

it's been a while, but on a 28/29 i believe the radiator and gas cap look the same and have the same threads....except the radiator is not vented and the gas cap is vented?

 

^^^^Yes, the gas cap will be vented.

 

Occasionally the center locking lug portion of the fuel cap will spin free once it is locked in place, you don't want that, you can not remove it!

 

On a '29 aren't the radiator caps a half-turn, and the gas caps fully threaded?

 

according to Snyder's the same except gas cap is vented

gas cap: https://www.snydersantiqueauto.com/1928-29-gas-cap