Honeywell Valencia rad valve problem
 

I had an unpleasant surprise when I removed a TRV yesterday - water
started to come out, as did the valve pin. Replacing, and screwing down,
the TRV stopped the leak.
There's a thin white plastic ring around the pin but I can't tell
whether it's broken off from something inside or whether it needs to be
screwed back into the valve body. I can't find a drawing of the valve
innards so I'm hoping that someone here has had one apart and can explain.