I did order one of these.Its recommended adding fuses in as there wasnt any from the factory so the cables end up being the fuse. https://pinwiki.com/wiki/index.php/Williams_System_3_-_7#Additional_Fuses
The board linked above includes that mod already done I believe.
Interesting, not seen that one before, but you need two I think, unless they come as a pair.
The websites on the PCB Dave.That's really useful, where do you find it?