Comes from Dashiell Hammett (who, of course, had his own Hellman connection). You'll find it in the Continental Op story, "Golden Horseshoe."
"I was reading a sign high on the wall behind the bar:
ONLY GENUINE PRE-WAR AMERICAN AND BRITISH WHISKEYS SERVED HERE
I was trying to count how many lies could be found in those nine words, and had reached four, with promise of more."