Hi Pollock,
I agree with Fido.
The part number you want is 67-3034 which is a 5/16x26TPI (cycle thread) thin wall nut. This means it has a smaller hexagon than normal which enables it to be tightened with a socket.
These are readily available from most of the UK dealers but I suspect that you are not in the UK.
Let us know where you are and maybe someone could suggest a specific dealer for you to try.
Beezageezauk.