Original Craftsman, Poulan, Husqvarna Flat Idler Pulley Part Number 173438, 131494.
Deck Idler Pulley and sometimes used on drive belt. ID 3/8", OD 3 7/8", 3 1/16" OD flat (where belt rides), 3/4" flat height. Used on many 12 through 22 HP Craftsman, Sears, Poulan, Wizard, Husqvarna, front engine tractors.