|MadSci Network: Engineering|
I have searched high and low for either an electrical or mechanical solution to this question and have come up dry. The only viable answer I can come up with is that the holes in the plugs are what they use in the injection molding process to hold the metal parts in place when they fill the mold with molten resin, and again when the mold needs to be released the holes can be used to hold the part in place when it is being freed from the mold.
If you look, older plugs don't have the holes and still work perfectly. There is an electrical reason for one electrode to be larger than the other, and if you'd like that explanation, ask me again.
Thanks for the puzzling question!
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