Well,
I left a part in some vinegar overnight. Worked a treat. Rub with a toothbrush, rust gone!
Today was shopping day. At 13p a bottle, I stocked up..... Other parts are pickling now.
Dean, Nick. Not sure I understand your gasket theories. Sorry!
The port tube (if it is a through tube), is threaded at 90*, through the joint, but offset by some 75%.....
Sorry, the female thread is out of focus!
I think this is the other end of the tube, and should have come out first. But, it won't unscrew, and the brass plug was a press (hammer?) fit.
Guess it will need drilling out.....
Things should be clearer tomorrow, when the vinegar has done it's job.
David D