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As by now you will have guessed it is a non-standard thread.
You can get specials made but you will need a mortgage.
If you are competent with a lathe you can screwcut the threads, from time to time 20tpi internal and external chasers appear on eBay and will aid finishing to a good profile. You could even use 1.25mm pitch, (0.050” = 1.27mm), close enough if needs must and particularly if you have a metric lathe. Generally it is easier to do metric threads on an Imperial lathe that English on a metric. I’d be inclined to first make a tap out of silver steel to finish the nuts, (first partially screwcut the threads). Then you can make the studs to fit the new nuts using them as go/no-go gauges, that way if the threads finish a bit off standard who cares as long as they fit? They’ll never see it from the High Street!