Hello and welcome.
It is impossible for a coin to be "stamped only on one side", just like it's impossible to clap with only one hand - the design is impressed onto both sides of a coin at the same time as it is squeezed between the two dies.
What you have is either a coin that has been so heavily worn that there's absolutely no trace left of the design on one side, or you've got normal halfpennies which somebody at some time (probably a long time ago, when the coin would have been found in change) and ground off the design on one side with sandpaper or a lathe.
Why would someone do that? We can only speculate. It was fashionable in those days to grind off one side of a coin, and engrave your initials or some other message on it, and give it to a loved one. Such "love tokens" are considered damaged coins by some collectors, as collectables in their own right by others, but a "blank love token" is just a damaged coin, in anyone's eyes.
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