Washer, Reducing, Galvanized Steel, Size 3 1/2 Inch – 3 Inch
https://hotfix.nsiindustries.com/product/washer-reducing-galvanized-steel-size-3-1-2-inch-3-inch/
The Bridgeport 3-1/2″ to 3″ 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.