Morning,
the BH 9mm cylinders headspace off the mouth of the case.......like a semi auto does. using an ejector rod to extract the casees, you don't need a "rim" to grab hold of like a DA revolver needs to......the moon clip gives us that "rim" to grab old of to extract. The ejector rod pushes on the case to extract.
using moon clips on the BH would kind of be a pain. you'd have to remove the cylinder, put the loaded moon clip in, reinstert the cylinder. then when through shooting, you'd have to reverse thos steps.
or at least that's my understanding,
~c.r.