Anyone ever heard of a thread called a PG7? It is on a plastic strain relief/entrance pigtale for a power cord on some studio lights. It seems to be close to 1/2-28 but is actually a little smaller in diameter. I have a dozen of these lights and they have all been assembled with 1/2-28 nuts and they are all stripping off. Only about 0.005" of thread engagement.
I believe I have a box of matching nuts comming from the manfacturer (www.heyco.com) so I don't need help finding them, I'm just curious as to what the heck they are and why in all creation did they use such an oddball size. There seems to be a whole series of PG threads as they list others - different diameters I guess. Anybody ever heard of them?