Missing bolt from 68 302 auto trans filler tube, what size ?

Hey Cougar people, I have a missing bolt and it used live at the back of the head on the right hand side (if you sit behind the wheel). It holds the filler tube still.
I need to know the size, is it course thread or fine. I am in South Australia and should be able to buy it from a fastener supplier when I get some other stainless bolts for my headers.
Would appreciate some experienced knowledge to educate us Aussie cougar apprentices.
Cheers
Leigh

3/8" X 16 NC about .500" grip length. It has an integral star washer because the same bolt is used for the engine - body ground strap.

Thanks Royce good tip.
Cheers
Leigh