How are Calor cylinders refilled?

Discovery4

No, no, I'm not intending to try to refill my own Calor cylinder, and would hope this doesn't drift OT into the rights &
wrongs of that.

However, I'm interested : how are the cylinders refilled by Calor?  Do they screw a nozzle onto the same thread that we then use to draw the gas off, or is there some other mechanism?

Reason I ask is that I've just connected the one I bought last week, and had to take a set of mole grips (carefully) to the on/off wheel because some numpty had screwed it so tight it was impossible to budge manually.  I'm intrigued whether it was whoever
refilled at the Calor depot (in which case time to rant next time I'm there), or whoever had the bottle previously.