You should only have to do one side, the side with the little depression
where the ejector pin pushed the knuckle out of the mold.

I will note that I don't (currently) use a knife for this but I use a
small fine needle file. No cut fingers!

I've not really had any flash in this area, mostly what I find when I
do find something is that the very pointy end of the tang has some extra
material that prevents the knuckle from fully opening (or closing, I
forget which), and this is fairly rare as well.

I was thinking further to my idea and instead of trying to drill a
shallow hole I'll find something, like sheet styrene whose thickness
matches the depth-to-tang measurement, drill a hold all the way through
and then screw or glue that to a piece of wood or something.

Tim