What Ragnar says about the little dimple in the receptacle's contacts is true.

However I've noticed this is mostly the case with cheaper connectors that rely on the spring action of a singlemetal blade and the opposite wall of its plastic housing. So the plug pin is held between one strip of spring-metal and a plastic "wall". The dimple will "lock into" the hole in the plug pin to help hold it in a bit.

These "single-wipe" receptacles are normally found in cheap household extension cords and the single contact tang looses its spring-action quickly. The plugs tend to fall out, wiggle and loosen.

GOOD quality receptacles will rely on spring action of two (or even three or four!) blades. These are called double-, triple- or quadruple-wipe contacts and do not necessarily need the little hole.

You will commonly see double-wipe contacts in cheap residential-quality sockets and triple and quadruple wipe in a lot of industrial and better commercial quality wall receptacles and replacement extension cord connectors. These hold the plugs tighter and last much much longer. Sometimes they hold the plugs so tight you need a good amound of pull to get them loose.

That's a good thing. [Linked Image]

Pins and contacts for molded plugs and connectors are stamped out in strips that are then broken off when the wires are crimped to them and before they are ready for the rubber to be molded around them. If you look at a transparent molded-on plug, you will see what I'm talking about.

I don't know what role the holes would play in this...and why the Australian and Chinese-market parallel-pin plugs don't have those holes when they're manufactured the same way pretty much.

The hole is optional.