A forage cap is actually one of those jaunty little jobs that Gurkhas and some members of the Rifles wear!
I always thought that the Gurkhas wore a 'Pillbox' hat. I believe the forage cap is the 'chipbag' style of hat like my father wore up to the early 1950s when he gave it to me to play with.
'We knew how to whinge but we kept it in the NAAFI bar'.