That's torn it.
This happened as soon as I tried to prise it off the barrel.
An
intact plastic ring has three tabs that need to be lifted at the same time to release it from the lip on the metal barrel. They wouldn't budge on mine using a fingernail so I eased a small flat-bladed screwdriver under one of them and ....
Ping! .... off it flew.
And the barrel still doesn't have a key entry cover.