The initial run of M&P 15s had serial numbers beginning with the two letters SW 00001.
However, right about the first of 2009, they changed the two first letters to SM 00001
This year's run begins with new letters, SM 00001 instead of SW.
So, if you've recently picked up a new M&P 15 and read about the early ones, it can cause unnecessary concern.
I bought a new one in February and already went through this worry step, only to find this information out, so I wanted to share it for those it may also affect.
Mine is # SM 0022x. Nawwww, it isn't QUITE the magical number of SM 00223 but it didn't miss it by much!!!