There may well be issues in the stihl literature about those clutch drums. My 251 has the 1123 007 1030 Sprocket kit picco 7 on it that has been stated on this site was/is good for the 250 on here and comes up as such when putting that number into google. The literature indicates the 231,241,251 are dedicated pieces and the six tooth spur has no part number and doesn't exist yet but if you argue with the guy, not the girl at the John Deere dealership eventually I ended up holding such an item. What are those letters your mileage may vary.
I see 250 at 10.1 pounds and 251 at 10.8 pounds power head only
http://www.stihlusa.com/products/chain-saws/homeowner-saws/ for the non easy start, non easy chain tension versions.