From The Vault: The Incredible Lightning Link
It took absolutely no persuading to get Brownells Gun Techs™ Keith and Caleb to return to Rock Island Auction Co., and today Keith has..... a machine gun in his pocket.
OK, not exactly. But he does have an SWD Ligtning Link Automatic Connector, an dirt-simple device that can turn your ordinary semi-automatic AR-15 into a machine gun, after the successful filing of your Form 1 and $200 check with the ATF. That was before May of 1986, when civilian manufacture of new machine guns became illegal.
Just like complete machine guns, existing Lightning Links at the time were grandfathered, and their value has appreciated - to between $10,000 and $15,000. The equivalent value of a complete machine gun for a tiny device made of two pieces of ordinary sheet metal!
The good news is a Lightning Link is the ATF-registered part and is not "married" to an individual AR-15. So if you have 10 AR-15s, you can shoot ANY one of them with your Lightning Link. But good luck finding one: only about 900 Lightning Links were made by SWD, Inc., and registered with the ATF. Which may explain why Rock Island's example sold for $18,400!