The bike is an 80's steel construction, so it's highly likely these dropouts are brazed into the slots in the tip of the fork tines.
This ride has survived 40+ years, so clearly it is "strong enough"
The cause will be slightly insufficient bronze filler used while brazing, or possibly shrinkage pulling the filler metal inward on cooling.
To relax your concerns, try removing the wheel and cleaning both dropouts with warm soapy water. Then dry with warm air. Once completely dry, use a tiny sliver of paper to measure the "depth" of the opening. I expect it to be millimetres at most.
A visual-only fix would be to use plastic filler or bog or epoxy to close the hole, then sand and paint over it. If you can't match the paint, then a small plain matt-black area over the dropouts would look fine too.