Older extinct threads on our sailboats..

Apr 30, 2025
4
Albin Vega Dundalk
So i pulled off the gate valve that feeds the toilet. Take it to the hardware store and it looks just like standard english thread on the ball valve at the store.
Not.. the new valve started to screw on but locks up after 1 thread.
What is the thread on the thru-hull to the toilet?