I just returned from a large chain bookstore where I bouht a Valentines card for my wife. they had a "S&W catalogue" for sale that I looked through and (book dated 2006) states that the M&P15 serial numbers begin with SW. Possibly the first batch of lowers were produced by an outside supplier and identified with the SW prefix. I know folks with M&Ps a few years old that have SN like mine with higher numbers than my (new in Dec 2010) example. I am pretty sure mine is new "old stock" and my ex LEO friend FFL dealer told me that Bud's , where I bought mine from, often buys large batches of weapons from other distributors or dealers who have financial problems. He could not buy the 15 OR from his supplier for what I paid including shipping from Bud's. He was good enough to transfer for me for 25 dollars, five of which went to FDLE for the background check.
I think if the equipment is correct on yours you should just relax and enjoy shooting it. The only ones I would not want to speculate on , quality wise, are the new low priced and down graded versions which some even state have some advantages (twist rate, questionable to me) vs. downgrades (lack of chroming on bbl. and cheaper components in some cases).
