The Husqvarna thread

IIRC, Stihl patented those toofed fasteners for use on chainsaws. Dunno if it's still in in effect or what. Dunno what fasteners are on the 5xx series Huskys....

572 fasteners are a T-27 Torx. The muffler bolts on some of the newer Huskies have that serrated finish underneath.

The downside to those Stihl fasteners is they chew into the plastic housings and casings on the consumer grade saws.