Green’s Dictionary of Slang

(Aus.) a sovereign; thus half a Yid, a half sovereign, ten shillings (50p).

1951		D. Stivens Jimmy Brockett 63–4: I got up and pulled out half a sovereign and threw it into the ring. [...] There was another shower of coins after my half a yid.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.