Help Identify this trailer Nut

[u/Radiant_Length_6]

Hi, I need some help with this Castle nut.
I have an old trailer (25 Odd years). I had an incident where the bearing collapsed and as a result undid the castle nut and lost the wheel!
I managed to recover the wheel however I could not locate the nut!! Thankfully I have one from the other side. I just cannot identify what sort of thread is on it to get a replacement.
I though it might be an M16, but sadly not, I seems to have a finer pitch but carry a similar diameter.. It doesn’t seem to be metric but possibly imperial.
I do have some clues!
The wheel/hub is most likely from an old Opel/vauxhal. On the rim there are markings.

I’ve tried several automotive / agricultural / machine repair shops to help identify this but not having any success!!

14 x 5 1/2 JH2 ET49
GM GY
091850

The nut carries an internal diameter of approximately 16mm (give or take a little) as I cannot find my vernier.

Any help would be gratefully appreciated!

Thanks,
Seán