I saw an old tin marketing sign last week that said something was 4d, and it brought back memories of pounds, shillings and pence, and the realization, much belated, that their abbreviations £ s d derive from Roman libra (= a pound), sesterces, and denarii.just as I had FINALLY figured out pounds/ shillings/ pence
You still have imperial measures in the US, and I imagine you have lots of oddities in it, like our old 'penny' being abbreviated with a d, rather than a p, because history embeds some things in a culture much more strongly than others.