To add some detail to this, the RS-550 motors do have two different product codes
M1-RS550-120 : This one comes with the 42mm gearboxes, and is NOT legal
M2-RS550-120 : This is the more powerful, FIRST legal motor from the KOP
The two look very similar at a first glance, but they have different vent patterns on the end, and the "M2" version has plastic fan blades attached to the armature, while the "M1" version does not.