The half crown was a remarkable coin, to outsiders, if you see it from their point of view.
It didn't actually indicate on it what it was worth, apart from being half of something that didn't even exist in normally circulation. Even if you, as a visitor, had come to grips with the whole pounds, shillings and pence thing, you still had to 'know' that a crown was five shillings, even though you would never seen a crown coin, and thus work out what this 'half crown' actually was.
I lived in a tourist spot in the 60s and remember often seeing American tourists (particularly) holding out handfuls of coins in shops, hoping that the assistant would remove the correct amount. We thought this was really odd, but they just didn't have the time to learn the system - and then there was also the fact that people used different word for things - pound/quid, shilling/bob, even sixpence/tanner, and you would still hear joey for threepence.
"Gee, pal, how much money is this 'half crown'?"
"It's two and a tanner, mate."