Yes, I'm pretty sure the 5/16x18tpi is for the Parker Hale Enfields, etc, whilst the 5/16x20tpi fits some of the Italian-made Enfield repros. My Euroarms Enfield Musketoon takes the 5/16x20. Never used the 8x1mm, but have a feeling Miroku, in Japan, used that on some of their Muskets. I've had a couple of Zouaves & shoot around quite a few others & they've all used the 5/16x24tpi nipples, even the originals....pretty sure it was the standard U.S. musket nipple thread.