So called “mad money” charms were a staple fixture on the charm bracelets of the mid-20th century. Some of them consisted of a precious metal (silver or gold) box with a glass/crystal window, some of them had a more cage-like structure, and some of them were like this lovely thing below, which is basically a Perspex cylinder finished off at each end with a silver finial.

I don’t know if anyone who wore a charm bracelet with a mad money charm on it ever took the pound note out and spent it. But back in the 60s or early 70s, a pound could go quite a long way – for example in 1970, a pint of beer cost about two shillings (10 pence after decimalisation). So your “mad money” pound note could have bought several drinks, plus food and a ride home.