Encyclopedia
The game of two-up
The Australian Oxford Dictionary defines two-up (or swy) as "a gambling game in which coins are spun in the air and bets placed on a showing of two heads or two tails".

Adelaide River, Northern Territory, 1943. Two-up games conducted
to raise money for the prisoner-of-war fund.
AWM 057097

Ypres, 1917. Australian soldiers playing two-up.
AWM E01199

