About 10 years ago, Mag Light changed their flashlights. The diameter of the barrel was reduced, and the switch was redesigned.
The old style is identified by a larger font of the serial number, about 3/16". The newer style is identified by a smaller font of the serial number, about 1/8", and a 'C' or 'D' preceeding the serial number. The old style takes a special wrench to remove the switch; the new style just requires an allen wrench; after loosening the switch screw, the entire switch slides out the front of the tube.
The switches are NOT interchangeable.
Hope this helps, Walt