Jet problems

The SS50 motorcycle engine is now pretty much run in, but the carb settings are a litlle of. So I thought I would try a different main jet but to my surprise the keihin SS50A carb is so old that it doesn't have the standardized jets still used today. Now available are jets with 4mm and 5mm treads, as far as I know M4 and M5, but mine has a coarser thread M5x1 ???
In any case it has the same threads as the jets used in the CB72 PW22hov carbs I have, the powerjet type.

some measurements:
5mm threads

6mm head

8mm long

and a view of the coarse threads

If anyone knows where to get these old jets, please let me know, I need something like a 75 or 78