Hi Roland, and thanks for the reply. I wondered if perhaps it was some whoop-de-doo mulching kit installed as an aftermarket "enhancement". OTOH, the spindle shaft is 1-1/4" long, and if I removed the short blade and accompanying spacer, it would lift the main blade 5/16" up into the deck, and expose extra spindle thread below the blade retaining nut. It is also odd that the new blade has 5/16" side holes on 2-1/2" centers to match the setup I have??
I attached a second photo to show the parts in better detail. Thanks.