Envision this: It’s 1917, at the height of World War One. Over a million U.S. soldiers are stationed in France, many of them armed with the workhorse battle rifle, the M1903 Springfield. The fighting is fierce and bloody, with both sides of the conflict taking heavy casualties daily. Then one day, a firearms designer steps forward with something that could drastically tip the scales in America’s favor: a quick-to-install device capable of turning those old hunk-o-timber bolt-action battle rifles with a five-round capacity into a fearsome semi-automatic with a 40-round magazine. That invention was known as The Pedersen Device.

Where you’ve seen it:

Not on screen. At best, behind glass at a museum. If you come across one of these in your granny’s attic, treat it with care—the last one that went to auction at Rock Island Auctions fetched nearly $50,000. Though 65,000 were built, almost all of them were destroyed following the war.