I got this from M1S a couple of years ago while doing my first build. At the time, I was told the carriers they were selling were like the old SP1 carriers. I know it does'nt have the smooth sides like the SP1's...but look at the rear of the carriers that are circled. The one at the top is the one from M1S and the bottom is a CMT from Del-ton. Notice how the M1S carrier is more squared at the rear and the bottom carrier is more rounded like most AR carriers. Sorry for the low quality pic...cheap camera. Any thoughts?

http://i43.photobucket.com/albums/e365/ken3983/DCP_00541.jpg

SP1 style in the middle:

www.quarterbore.com/images/ar15_sp1_m16carriers_sm.jpg

both of your appear to be "regular" AR carriers...?

Could they have meant SP1 style in the sense that the firing pin is exposed?

Most SP1 carriers have forward assist teeth even though the receiver is slickside.  Both of yours look close enough to use.