Don't loose site of the primary objective, that is having a secure hinge clamp.
Its apparent from the photo that there has been no metal to metal contact in the area of the hole since the paint is in good condition.
I would speculate that the area may be for a small piece of cushioning plastic. Dahon uses some HDPE on one there hinge types.
If adjusting the clamp spring tension with nyloc on the spring shaft gives insufficient tension , experiment with cushioning materials in the area of the whole.
I had a halfway but sold it to a friend several years ago, nice bikes