Buy a stock plastic or aluminum thread gauge and see which pattern fits the threads on the screws you have in hand. Buy both a metric and an imperial gauge to be sure to accurately size them up.
Dave
They are quite similar to those used in household electro, where the screw screws into a hollow plastic tube and "taps" its own thread in the soft plastic.
If they are not PT thread form, then the question about metric and inch is moot. The standard sizes for tapping screws like these are 2.9 mm, 3.5 mm, 4.2 mm, etc. These correspond with inch sizes 4, 6 , 8, etc.