I am a huge robben ford fan and as I have looked at youtube clips and other audio samples it appears most folks achieve "their robben tone" via an overtone standard special?

I am also curious about the overtone special FM as well...i think I have figured out the that FM stands for "ford mustang"......can anybody tell me the primary differences between an "overtone special 50" and an "overtone special FM 50"?
thanks in advance for your help.