Any markings on the bolt that indicate its a LMT im not sure what mine is.
From what i've read there is only one way to tell if its an LMT bolt carrier and thats the carrier staking. If its a horizontal staking (looks like its been hit with a flat head screw driver) then its a stag/cmt carrier. If it was staked with a MOACKS tool (sides pushed in with round pins) then its an LMT. S&W apparently only used stag/cmt and LMT carriers so it has to be one of those two.
Heres the link to a site that has pictures of all the carrier stakings from different brands: Bolt Carrier Stakings
FWIW any of the serial #s starting with SW that are 5XXX or above are S&W made guns, any before that were contracted out to another maker (Stag?)
and those are the ones that had the issues. The alpha prefixes change every 10,000 guns (actualy 9999).
Thats kind of what I figured, but I just wanted to let people know not to worry if they wanted to get an M&P15 but it has a low serial number. Like I said when I bought mine I was afraid it actually was an early build. It really messed me up when I was reading online about all of these S&W builds and people saying there serial numbers were like 05XXX and 65XXX but mine was 003XX.