JD Parts no longer gives us the info we need such as length, width, thickness, and hole size for the M82408 blade, which is the standard lift blade listed for both the serial number breaks. Also says sub M82408 for AM1009466. Weird! There are medium and high lift blades that can be used if you desire. Different part numbers of course. Then there are Gator and Predator brand mulching blades. But you will want to know what blade actually fits first.
Anyway, maybe you could take both blades to the dealer and see which, if either matches whatever part number they have for that deck. Take a copy of the serial number along to save trips.
While the blades are still mounted, check how much distance is between the blades when lined up at the closest point. Usually they are about 1/8-1/4" from touching.
I'm going out on a limb and thinking the 19.5" blade would be the correct one. 2 x 19.5=39". The blades usually overlap a total of 1.00" on JD decks I have worked with. That's supposed to keep from leaving uncut strips of grass when turning. On the 2 blade deck, the left spindle is set forward of the right one so the blades cut only a 38" path and don't hit each other. On the 3 blade models, the center spindle is forward of the other two. The decks that use timing belts are different, but I don't think that is what you have.
Keep us posted with what you find. Hopefully the deck isn't bent somehow and a previous owner put on a different blade to make up for it.
tommyhawk
