Why do you need T slot outlets for 20A split receptable outlets?

What is the purpose of power outlets with T-shaped holes? t slot receptacle receptacles? Any idea what years those receptacles are from? Also what type of plug fits into the horizontal slots of that t-slot receptacle? Posted By t slot brackets The receptacle looks slightly different from the 15A outlet with a small horizontal slot branching of a vertical slot These are becoming popular as you don't

the green knight slot soldi veri The T-shaped slot indicates that the 120-volt receptacle is rated for 20-amps Regular wall receptacles with a pair of vertical slots are