Are you sure it's an M16 carrier? The AR15 bolt carriers back in the 70's were not like the omes they use now. Back then, they were basically M16 carriers withe the underside milled back about 3/4 inch and the under the firing pin area of the carrier milled out enough so the rear of the firing pin shows when you look at the assembly upside down. Basically, they were M16 carriers with enough metal cut out to not work in an M16 on full auto. If that's what's in your weapon, you're OK. It's original. It may even have the bolt assist cuts in the side, even tho the SP1 didn't have a bolt assist assembly. Basically, if you can turn the unit over, and see the rear 1/2 inch of the firing pin, you've got an original carrier. If the entire firing pin asssembly is covered by the carrier, and you can't see any of it, it's an M16 carrier and you need to get it out of there.