I can't say for sure about the civilian versions, but the military M9 has a fixed rear sight while the M9A1 has a drift adjustable rear sight. The M9 I carried in Iraq shot a good 12 inches to the left at 25 feet. Nothing a little work with a hand file couldn't fix though.
Alcohol Tobacco and Firearms should be an aisle at Wal-Mart, not a government agency!
Only faithful men teach their wives to shoot.