Salt and vinegar?

Yesterday's post reminded me of another war story - WWII this time.  Don't know if it's true but I was told it by someone who was definitely part of the D-Day landings. He told me that it was accepted that in the immediate aftermath of the landings, there would be chaos and confusion, and so in order to be able to distinguish between friend and foe, you would shout out "fish".  If you got the response "chips" you would know you were safe - at least temporarily.