Green’s Dictionary of Slang

(Aus.) cards that have been shaved on the ends to make their denominations clear to the cheating player .

1882		Sydney Sl. Dict. (2 edn) 5: Longs and Shorts - Cards made for cheating.
1919		Mirror (Sydney) 31 Aug. 8/2: In banker, ‘longs and shorts’ are frequently, used. The cards are shaved on the ends, and the player knows when he is cutting a high card or a low one.