Sorry I cannot help with the origins of "pink chit", but it goes back a long way. The married members of OUAS in the late '40s (and there were plenty of married veterans in statu pupillariin that period) used to announce to us pimply sprogs straight up from school that "they had a pink to come out on the pi$$". The word "chit" I am sure is peculiarly British, with its origins in the Raj. As a child of the Raj I used to sign "chits" on my father's account for soft drinks in various clubs in Rangoon in '40/41.