RAT 5,
my copy of the FCTM states that "the autopilot is FAA certified to allow engagement at or above 1000 ft AGL after takeoff" (300 - 500) and "at or above 400 ft AGL (600 - 900)".
The UK CAA also says 1000ft for the 300 - 500, don't know about the NG.
Because of this the family of 737 probably will make a difference to company SOPs.