The MB1 was the first of the MB series and was replaced with the MB2 about 3 years ago.
The MB2 has the crossover and amplifier connections directly on the back panel of the speaker and has a curved front baffel for better imaging stc. The MB2 also has a much thicker front baffle to reduce resonance.
If it is a very early MB1 it may have the older midrange unit instead of the PMC midrange unit that all MB2's have. You can tell if it is the older mid because it will have 2 little wires coming out from under the mid-dome.
james