A while back I asked a question in this thread:
http://www.arboristsite.com/showthread.php?t=59011
The question was about the part number for the muffler with the large outlet, the one used before OSHA strangled the saw. No one seemed to know what the part number was.
After looking in the IPL I decided to order the whole assembly that included the dual port cover. I thought there might be a chance that they would match. The gamble paid off.
Part number 1122 140 0603 includes the large outlet muffler, the dual port casing, both spark screens, and the screw for the front screen.
A pic of the large outlet muffler
It would be great if Stihl would sell an MS660 Power Up kit, like this one:
Pictured are the:
Large outlet, dual port casing muffler (1122 140 0603)
Limiter caps (4203 121 2700)
Limiter cap puller (5910 890 4500)