Merry Christmas all, waiting for the guests to arrive and better do something or I'll eat all of the snacks put out....
Thinking about it I looked around and found the TR4a drain on one of my (3) TR3 radiators, I'd forgotten about that one as I've never used the rad.
It does fit, obviously, and is now in my ally rad. I did use the thread seal liberally on the threads so we'll see how it holds up.
Thanks all,
Pat