Hello, Highspeed5,
I've recently purchased a nearly new (fewer than 1000 rounds fired, per owner) M&P 15A in excellent condition. I am not aware that the owner was LE or anyone professional requiring carry of a MILSPEC or similarly-equipped weapon.
Needless to say, the rifle is a pleasure!
The main difference between the "A' and the "X", I believe, is the four-rail tactical hand-guard.
The rear site on mine, although labelled "S&W M&P", is a Troy Industries Folding Battle Site, apparently same as on the "X". The previous owner did not mention any custom modifications he performed, and photography of samples on the S&W Internet site shows same-appearing rear sites on the two models, although Troy Industries is not specifically listed as the rear site brand in the spec. sheet for the "A".
Since I have the actual rifle and accompanying paperwork, you may contact me via the Forum if you further have specific questions.
From my limited knowledge of the SWAT-type requirements, those personnel highly value a versatile platform from which to launch additional capabilities, and the tactical hand-guard, I suspect, would be a real "hot" item in those circles (pun intended, I guess)!
I would not be surprised that there is a real supply/demand differential between the "X" and the "A", with the brand-new former (released Dec. 2007) being channelled to the professionals and, therefore, being less available to the general citizen, despite the fact that they are both "Military&Police15...". If your friend is not LE, he may have to wait longer or "settle" for the outstanding "A"!
Hopefully, I've been able to clarify a little.
Best wishes.
Victor.