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

Leviton 5000-I Duplex Receptacle T-SLOT Residential Grade 1-15R t slot receptacle Don't have an account? Create an Account · Cart · Home · DIY Projects 20-amp receptacles have a horizontal slot branching off one of the vertical slots tslot milling cutter sizes Don't believe me? take the 15 amp and the 20 amp of same grade of receptacle and peer inside the wide slot, that's the neutral slot the T slot

triple double diamond slot machine free games 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?