Blackjacks were they used in ww2?

There was mention of a black jacks or slap jacks / flap jacks (leather filled with hard items) that may have been used in close combat during ww2 any more info?

 

Yes. The British Commandos favoured the hanging straps on London Underground tube trains. You can see one trucked into the webbing of Here is Lt George Herbert DCM, MM of the Northamtonshire Regiment, of No 3 Commando

 

Interesting. What about the US side? I believe the Police had them in the 1940s, so the army may have.