Due to the hyphen between the first two initials, I assume these stand for "Porzellan-Malerei", meaning "decoration studio". This would explain why these marks are often found next to original producer marks or in the form of cover-ups.

M100911a
Crown above the initials "PMB" on an item produced by C.M. Hutschnreuther in Arzberg.
(Picture: Kim Halley)