Wouldn't it be quite easy just to make a tap for that Ian?
5/16 silver steel, turn down and thread it male, grind 3 flutes into it and then cherry red heat to quench in oil.
I have done it many times to get an odd thread sorted and it works really well. A 55 degree point on a small hss tool is all that required for the screw cut.
Now it's not that I am frugal, I just like solving puzzles like that as an old friend says, if the sub is stuck on the bottom, failure is not an option.
Darren
