THE introduction of the new $10 coins has proved a yuletide bonanza for Wan Chai's girlie bars. Working on the W. C. Fields principle of never giving a sucker an even break, the tough as teak mamasans at some establishments have stumbled on a ruse to milk even more from the wallets of the sozzled or unwary. They are handing over the change tray loaded with the $10 coins buried under one or two notes. In the poor light, the coins can pass for a $1 or $2 coin nine times out of 10, and after as many drinks, the customer takes off the notes and waves the change away. Numerous Keeping Posted spies have braved the financial death traps of Lockhart Road and come out losers. One informant said many customers seemed unaware of the coin's existence. 'It's only when they get home they begin to wonder how that $500 note disappeared so quickly,' he said.