This chain is manufactured by STIHL themselves; and is designed to fit a 18″, .325 Pitch, .050″ Gauge, Stihl bar.
STIHL RAPID™ Super (RS) chain
For professional and home owner use, producing outstanding results. This
chain features a raised back of the cutter and tie straps to
reduce vibrations by up to 70%. This heavy-duty chain is
ideal for use on machines rated at more than 2.5 kW. Model in .325″, .050″
Often used on a MS261, MS271, MS291, and MS291C