This 50" GB Titanium bar is a 152 drive link .063 3/8 bar.  This will fit Stihl 880 saws. Adapters to fit MS660 are optional. For fitment on a MS660,661 you need to use 153DL 3/8 chain. 
 
Titanium® ProTOP™ Professional Chainsaw Bars
Features and Benefits:• One-piece Titanium alloy steel body• Laser-cut for precision shape and fit• Precision-ground grooves• Hardest wearing rails in the world• Increased flexibility in the bar body• Easy nose replacement• Nose side plates made of hardened Titanium alloy steel.• During hardening, the nose side plates are injected with Carbon Nitride, which guarantees that Pro-Top™ is the hardest-wearing replaceable sprocket nose in the world• Nose self-locks over a solid steel web on the bar body, held firmly together by three high-tensile rivets• Super-tough, high-tech alloy sprocket glides on precision Swiss bearings