What year is the mower? Is it the T2200? The thing is the OEM blade your mower uses can be any of the ones I listed in the previous post and they will fit a number of different mowers.
I'm not sure if it's the T2200 - I bought in May 2014 and this link seems to indicate it's the T2200 because it says 20381 which matches my manual @ 917.20381
2014 Craftsman T2200 Model 20381 42 in Hydro 19hp Yard Tractor Review – 540cc Single Briggs Platinum | TodaysMower.com