According to
this article, they were used when the Mint was still situated on Tower Hill in London. As that mint almost exclusively struck predecimal coinage, presumably they are denominated in predecimal rather than decimal pence, and would date from the 1960s rather than the 1970s.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis