12 pence
I was told about the silver sixpence story by an old time aviator, when there were still lots of them around in the crew room (the type who would smoke a pipe in first class, at low level ). Unfortunately though I never found out why, just that it should be done. I have a silver sixpence behind my flying badge, the one on my No 1 that is (and it still fits).
It (the flying badge) was also dunked in champagne, Vintage Dom P......53 I think, same old geezer told us scrotes at the time of it being traditional too. Laterly I have seen, at various grads, students discuss the silver sixpence thing and wander round antique shops trying to find one. The dunking thing now seems to be done in beer or even worse lager.......just no style left really.