Andy,

1/4 unc or whitworth will do, maybe you could talk Pat into sending you some threaded insert adapter that go into the bottom plinth, that will give you 3/8 UNC like the shearwaters, I've got plenty of threaded adapters and cones you can try then.

M/L Martain Logans= A quick route to terminal boredom via flight yawnsville

M/L Mark levinsion= Quality built and accurate reproduction equipment that pairs well with the above.

M/L Meadowlarks= Exceptional good musical speakers, that won't be for every one.

M/L Merlins Ladehausian= Shawn off leather trousers for our Z/G wizard to go with his matching braces