What I meant was one of these:

Those were cheap, mass produced clickers or 'crickets' that were supposed to help to identify friend from foe. If you heard activity nearby you gave two clicks,and if a response wasn't given, you'd assume a foe (the call sign 'flash' and response 'thunder' was also used as they are shibboleth , hard for Germans to pronounce).

Ofc it isn't an original one, but absolutely historically correct, in that they were cheap and mass produced. It's proved to be a conversation starter and thus absolutely worth it's price.