That's odd. Back in the old days when drop in auto kits were legal & a dime a dozen, the ad specifically said it was plug & play with no alteration required to the gun. Someone was lying?
You must be referring to the SWD Lightning link...
Without a lightning link "select fire kit" when the LL is installed it's auto when you pull the trigger. It was designed to be used with certain receivers they had when it was introduced and only works with certain carriers. You can read up on it here.
http://www.quarterbore.com/nfa/lightninglink.html
Down south the three different kinds of M16 mg's are the registered receiver, RDIAS and LL.
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