539101080 Husqvarna clutch interlock switch. This is a plunger style switch with 2 positions and 4 terminals. 
Will also fit the following models: Ariens00191256, 08828100AYP109553X, 539101080Bobcat108208Briggs & Stratton1717050BuntonPL7484Bush Hog99073Delta6400-53Dixie Chopper500019Dixon4242, 539101080, 539115802Encore523034Exmark1-633111, 633111Gravely08828100Husqvarna532109553, 539101080John DeereAM103119, AM115055, AM1288925, AM128925, TCA20554Kees101080KubotaK3211-62251MTD00012242, 725-04039, 725-3169, 725-3169A, 725-3169AP, 925-04039, 925-3169AMurray91299, 91299MANational7325Scag481545Simplicity1717050, 1717050SMSnapper1-7590, 2-3355, 7023355, 7023355YPSteiner31-061Toro112-0455, 1-633111, 86-4690l Troy-Bilt 00012242, 1751328, 725-3169AP, 925-04039, 925-3169A, GW-1751328l. Woods 071148, Worldlawn 523034