BTW, the holes in the hinge flats came about in the early days of flat, pinned hinges. They were originally epoxied into the balsa. (And that was a mess and a pain to keep the hinge joint free of epoxy, but that's another story.) And turns out that the hinges would let go after awhile sometimes.

They discovered that the plastic in the hinges was releasing itself over time from the epoxy. So they drilled holes through the flats so the epoxy would "pin" the hinges into the slots. Dubro also changed the plastic they were using.

Hinge glue bonds to the plastic being used today. Bonds good. Plus the holes are still there in the hinge flats.