when originally released they were released as mumbos and came in e colours, black, white and neon yellow.
they only came with 3 lense shapes, hybrid, 67 and v-lense.
after a dispute with the mambo clothing company, oakley then renamed them m-frames which is where it has stuck to right up to today.
so if you have a pair with black, white or neon yellow frames with any of the original 3 lenses with oakley down the side of the arms-these are mumbos, anything else in another colour or other lense ie heater, strike, hybrid, strip sweep etc are m frames!