As far as I am aware, all of the current Fuso wheel nuts have a M20 thread.
18.5mm is not a "standard" metric thread size, so it may be that this is a propriety thread. If you have access to a metric thread gauge I would also confirm the thread pitch.
If your studs do have a non standard thread size, your only real option may be to go to an engineering shop and have some wheel nuts custom made.
One positive is that you have a sample that could be copied, but bear in mind that most things that are custom made come at a price.
If I were making these I would use something like 4140 steel (or equivalent), which would be more than adequate for the task, then have them zinc plated.
Machining an internal thread of that size would not be overly complicated, be it left or right handed, but that would be the only option if a suitable tap could not be sourced.