I always thought it was "Ace Under A Tack!"
I was never sure who Ace was or why he wass under a tack - perhaps some sort of perverted P38 drivers who had lost their landing gear again and are seeking penance like wearing "ashes and sack cloth". Or maybe something worse.

I had visions of a pair of blurry beedie eyes peering up from underneath while muttering " Have all the HO tards gone home?"

............you never can tell ....one of lifes mysteries eh!!
