If you enjoy the forum please consider supporting it by signing up for a NES Membership The benefits pay for the membership many times over.
Be sure to enter the NES/MFS May Giveaway ***Canik METE SFX***
Minor differences between the rear sight, dust cover, and maybe the blackstrap I believe.
Here's what one gun blogger posted recently, although he doesn't give any source of the claim.
http://www.gunnuts.net/2014/04/04/photo-of-the-day-beretta-m9a1-frames/