Why do some receptacle outlets have a T-shaped hole?

slot in the middle, and a vertical slot beneath the triangular slot I can't make out any numbers embossed on the outlets, can you see any?

These differences don't change In North America, this is accomplished by having two differently sized slots in addition to the rounded grounding slot

totem towers slot On a 20 amp receptacle, one of the two flat slots will be T shaped, rather than just a straight slot On a 20 amp plug, the blade for the hot

the angler slot EATON 20 Amp White Decorator T-Slot Receptacle · $ · $ · Product Overview Description From decorator and standard to tamper and weather resistant