Synthesis and Chiral Resolution of C3-Symmetric Tribenzotriquinacenes, Eur. J. Org. Chem. 2016, Early View article
I came across this graphical abstract with M and P overlayed. After reading the article it became clear that M and P were being used as stereochemical descriptors, presumably of some kind of axial chirality/screw sense.
When I've previously looked, I've often seen either (+)/(-) (where + is the right-handed screw and - is the left) since this is actually describing the physical property of the molecule as measured, in the same way that (+)-proline describes the way in which the molecule interacts with plane polarised light. In many cases (R) and (S) are also used to describe axial chirality (BINAP, for example).
A quick google of the issue seems to suggest that (-) --> M and (+) --> P (cf. Wikipedia, confirmed by the IUPAC goldbook), but I was wondering if there is something more systematic going on here (i.e. a (+)-molecule doesn't always have an (R)-configuration).
In my head, the fact that there are descriptors like M and P (cf. E/Z and R/S) suggests that one should be able to look at the molecule and assign these by some kind of priority rule rather than by physical measurement (and by extension, does M/P actually predict +/- in a way that R/S doesn't).