Idaho guy

Greetings all, my first post, and I have a question: For the Surefire light forearm for the Benelli M1 http://www.surefire.com/dsf-m1-m2-shotgun-forend.html , they are not suppose to fit the M1 with serial numbers below M104800. Anyone know why this is? I have an early M1 Super90 and I'd love to have a forearm with a built-in light system.