PZM (pressure zone microphone) or Boundary Microphone (hence its name BM1).
Omnidirectional semi-hemispherical. Made in China by LD Systems.
Miking coherent avoiding occasions of direct sounds and reflections, generating comb filters.
Powered electret phantom power 48V, output and mini-XLR 6m cable (ultra thin) finished in XLR.
Crown PZM of the patent fell into the public domain and there are several brands of "Boundary Microphones" quality at affordable prices.
I bought a pair of Realistic PZM time when there were still Tandy stores in France ... Realistic and these little cheap to me often favor.
I took these LD System for smaller microphones (yes, a cajon was not always a vent big enough ...).
Sound recording of meetings, recovery of jazz drums (2 PZM, made very close to the direct sound), and the classic bass drum and cajon recovery.
I do not regret having invested the few dollars (the 1/4 the price of a beta91).
Only complaint, the cable supplied is 6 meters and it is fine as a hair. I agree that it's quieter in the middle of a meeting table, but this cable will not last long. I have a shortcut to 30aine centimeters, and I put a "real" XLR cable as a result.