My understanding is that nobody knows the specific serial number range, but anything made after March 1, 2014 was made after the fix for the veiling flare was implemented. The plane, gray box that is outside the decorated Leica box should have a sticker on it listing the manufacturing date. Failing that, you can call Leica with the serial number to check whether it is safe or not. Note that not every lens made prior to 3/2014 had the flare issue, so even if you have an earlier lens you don't necessarily have a problem--just test the lens to see. A backlight subject shot with the lens fully stopped down should tell you. Also, keep in mind that ANY lens can flare under certain circumstances (including the 50 APO), so don't stress too much if you get the occasional bad frame. It should be rare.
- Jared