They are only different in the way the lense was assembled. They are interchangeable as far as appearance and installation. Wiring and mounting holes are both the same. However, the reflector in the center of the lense on the early 240's are glued in with no backing. This is only significant because the glue will dry and the reflector can fall out. The later 240 lenses also have glued in reflectors, but they are glued up against a solid plastic backing. The glue bonds them against the lense so well, they will shatter before popping out. Both look identical to the untrained person.
(This sounds pretty obscure, but you'll find all this out if you have an early one and the reflector disappears one day for no apparent reason) - Chris.
