Just heard this week that the 9640 003 1850 Oil seal 17x32.9x3.6 (PTO side crank seal for 064, 066 and MS660) is NLA on its own from Stihl. It is, however, still available as part of the full set of gaskets and seals, part number 1122 007 1053. Apparenetly as the MS660 has been out of production...