Product review (2 stars): Not a genuine Whirlpool part - there are fit issues.
This is NOT a genuine Whirlpool part even though it is advertised as a "Whirlpool 285811". It doesn't have the "FSP" logo that the genuine parts have.  As such, there were issues.  The thrust washer was a little too thick, such that the upper and lower parts of the agitator bound together once I tightened the nut.  I had to reuse the old one.  Also, the O-ring for the top cap is too thick, which prevented me from installing the top cap, no matter how much soap I put on it to try and get it to slip back on.  Again, I had to reuse the old O-ring.  So, the only parts that I actually needed to replace that I could use were the agitator dogs themselves.  I bought the whole kit so I could do a more thorough rebuild, but in hindsight I should have only bought the agitator dogs.  The machine is working again, but if the other parts had actually been bad and could not be reused, I would have been screwed.  At least the price was right and it was very promptly shipped, which is why it got two stars instead of only one.  If this happens again, I would only buy the agitator dogs, which is probably the only thing that actually really needs to be replaced.