Punch up the bracket

what is the origin of the phrase " a punch up the bracket "

For our non-Cockney visitors, It means a punch in the face but I do not know the origin.

I remember the comedians Tony Hancock and Sid James using this in their radio shows. It was in common use when I were a lad in the 1950/60s. I would suspect it is British army slang from WWII but have no proof of that.

Gary