I have a 1956 set of slings. When reassembling the set, I find I have two lugs which are unlike any of the lugs in the set. These two lugs appear to be identical to the rest of the lugs from the outside. However, when you turn these two lugs over and look inside, the wall of the lug is thinner than a regular sling lug, the spring is thinner than a normal sling lug spring. These lugs also do not have the shell screw hole extension. In a normal lug, the lug fits into the shell via extensions that fit into the shell as guides. These two lugs do not have the extensions. Normal lug screws do not thread into these lugs easily. What is the origin of these two lugs? (I am not the original owner of the set.)