Galvanized Steel Reducing Washer 3″ to 1/2″
https://nsiindustries.com/product/galvanized-steel-reducing-washer-3-to-1-2-2/
The Bridgeport 3″ to 1/2″ Galvanized Steel Reducing Washer is used in pairs to reduce the size of knockout holes in a box or enclosure. The washer features raised flanges that allow for positive seating in the knockout.