We got a nice pile of replacement caps for US canteens. These also fit other canteens, or not. These might also work as fidget items; put the neck connection ring on your finger, and spin the day away.
As far as we can say, this cap fits threads that are 1.5" / 38 mm wide, such as the Dutch surplus canteen with the black cap. The green capped Dutch canteen has 35 mm threads, which just is not enough. Size does matter.
Another idea: If you are looking for a military-themed shot glass, this is it!
This cap is plastic, OD green. The canteen neck connection ring is integrated, so it turns with the cap. The ring has an inside diameter of 1.4" / 35 mm.
These caps are unused US surplus.
Ashish B.