It's the bolt that holds the rear brake hose and provides ventilation for the 8 inch differential. Mine stripped on me and I'm having kind of hard time finding a replacement.
Leave it to Ford to make different parts, no idea if they interchange. 10" brakes, 5 Lug --- D1OZ-4022-A 9" brakes, 4 Lug --- D1OZ-4022-B
For only $141.00, you could have a 2500 year supply https://www.ebay.com/itm/1557598739.../vZcnUxiJ9tg/rn7fSCoJaY6fojy|tkp:BFBM1s2F_Pxl