Could it be MKIIIN? That could be a starting point for your research. The font reminds me of British markings on various items, gas mask components and such.
I just found this entry for a '942 Mk3N fuse' used by the British, no pic though yet but it does have some dimensions of the fuse. Maybe they will help you determine if this is a cover for it.
Fuze 942 MK3N | Imperial War Museums