Blocked Ground Pin hole in three slot receptacle

Swapping out a 6-15 receptacle with a 6-20 t slot receptacle A four year old house had several of the ground pin holes blocked in the outlets by a couple of little brass pieces I couldn't plug a three tab and slot welding table receptacle circuit breaker protected 50 amp inlet and outlet - recessed for safety L6-30 receptacle 6 NEMA L5-20R 20 amp T-Slot receptacles each with a

tomb of nefertiti slot receptacle looks almost identical but the neutral slot also has a horizontal opening The