|Compo Can Openers|
It came in a greaseproof envelope with blue writing and was called 'The Baby Can Opener'. Not only used for opening cans, it was the first multi-tool that many soldiers had, being used as a bottle opener, sharp point, (very) small knife and screwdriver. It got sharper and the useable blade length increased with use. There are probably many other uses and contributors can feel free to add any that they used it for.
Indispensable item on your key ring or tied to the end of your Racing Spoon.