Both the James ML and Royal Enfield Flying Flea used this rather large, Bakelite plug cap........."Lodge" brand and essentially a pre-war 1930's type, very "bell-shaped" and NOT suppressed in any way...........

These are now rather hard to find but do sometimes turn up at autojumbles and Ebay...............

I found out today that if fitted (as in the case of the ML) to an original Lodge C3 spark plug you need to "reverse" the wide knurled brass nut on top of the plug so that the narrower portion is above the "knurled" hand-tightening section...........that way, this Lodge plug cap securely clips firmly into place.............