The nipples for reproduction arms are a 1/4" square drive in various thread sizes and types. My ArmiSport P53 Enfield uses the same nipple as a pre-1996 Parker-Hale which is a 5/16"-18. The Parker-Hale muskets made after 1996 are supposed to take 5/16"-20 nipples. Be advised, some nipples that size will fit the threads but will not fully seat which depends on how deep the nipple threads are cut in the rifle. After I made an thorough search and purchase of six or seven different 5/16"-18 nipples of different types, the only one that fit and fully seated in my ArmiSport P53 Enfield was a 5/16"-18 Dixie Gun Works NP 1430-steel nipple with a 1/4" square drive.
The supplier where I bought my P53 Enfield said it took a 8MM x 1 nipple but he was wrong. Even the person that defarbed my Enfield said it took a 8MM x 1 nipple and he was also wrong.