In 1941, Aziraphale tries to do a coin trick for Crowley and the two coins he uses are a sixpence and a farthing. You could headcanon that this is something they both carried after the events in those episodes.
I used Procreate to carefully combine these two coins to make the fictional one above. When I discovered the farthing had a bird on it (a wren), I got the idea to make it a nightingale. And luckily, this German euro coin has a singing nightingale on it.


















