Mr Heater Buddy help

This rubber? ring fell out of where the propane canisters screw into my Mr. Heater Buddy heater, I'm wondering if it's a seal of some sort or just built up gunk? I haven't tried using it since it fell out, is it easy to replace/ repair or do I need a new heater? What does urs look like if you have it on hand? I tried googling replacements but didn’t find anything