You need the seventh part ( an M2 hammer ) to make it an unregistered full auto kit. Six out of seven , a collection of parts. Seven out of seven , jail time if not papered. Any reciever marked M2 , or an over stamped M2 , or one in that serial number range of those with the number removed , are treated as full autos no matter if they have the FA parts or not. Ie , must be papered. FA triggerguards , slides, stocks , sears and round bolts are not kit parts and are not considered.
HTH , Chris