MPS and MPST "darts" for APS rifle?

Hi all; I’m looking for some specific information of the MPS and MPST 5.66x39mm “dart” rounds, as used in the APS underwater “rifle”; Bolotin’s book explains that the “PS” and “PST” in these rounds’ name stands for “podvodny spetsialny” and “podvodny spetsialny trassiruoshchy” (“special underwater” and “special underwater tracer”; an underwater tracer round :-); how’s THAT for unusual?), but doesn’t seem to explain what the “M” in the name of either of these rounds means. Does anyone out there know?