I recently had the opportunity to service a pair of MB301 cardioid ribbon microphones. I had always understood these to be rebranded Beyer M260s with a custom grill, but this is not the case.

The MB301 does use a Beyer pistonic ribbon, but the motor is completely different. The Beyer M260 uses four glued rectangular magnets to make the motor,  but the MB301 employs a single large cast ferrous horseshoe magnet. In this respect it looks more like an RCA BK5 or Oktava ML19.

MB301 (left) and Beyer M260 (right)

I think it more likely that the magnet is taken from the Beyer M320 / M360 models, with different pole pieces, although I have not had the chance to compare these on the bench at the same time. Either way, the MB301 is a microphone in its own right, and not a copy of something else.

Despite the ugly oversized grill, they sound pretty good with plenty of top end and bass, and the big magnets give a stronger output signal than the early M260s.