Green’s Dictionary of Slang

(Aus.) a betting shop, offering only starting prices; thus S.P. money.

1929		C.G. Gordon Crooks of the Und. 84: You can imagine the amount of S.P. money he takes every day [...] he’s got a lot of punters who have ten pounds a race.
1938		Smith’s Wkly (Sydney) 26 Feb. 3/3: [T]hey took a taxi to some S.P. joint, walked in, pulled guns and grabbed the ‘bank’.
1945		Baker Aus. Lang.
1955		N. Pulliam I Travelled a Lonely Land (1957) 238/1: s. p. shop or joint – a starting-price betting shop.