For those that own an M1A1, this is a pretty neat accessory to have. Mark Bando's website below (scroll down) details two types that were used by actual WWII paratroopers. I have a type 2 and here's a link to a type 1 for sale on gunbroker. Not cheap, but they are seldom seen anymore.
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Rare WWII Paratrooper M2 Jump Knife : WW1 & WW2 Collectibles at GunBroker.com