For classic era guns:
A is in 9 mm Largo, for Spanish military and paramilitary issue
B is the export version of that in 9 mm Luger/Parabellum. These two are identical otherwise, you can
P is a larger frame, but similar, in .45
M is the same frame in a bunch of calibers other than .45, for no clear reason.
Insofar as anyone gets it, understanding model numbers and suffixes
http://star-firearms.com/firearms/numbering.shtml
For example, there's another model "PD" but it's rare so mostly not confusing.
How to get your (more or less) date of manufacture:
http://star-firearms.com/firearms/proof.shtml
Ps
http://star-firearms.com/firearms/guns/p/index.shtml
PDs were a classic. The compact before compacts much existed, so was small, light, reliable, readily available and cheap when the competitors were fewer of those. It's the gun for which "carried much and shot little" was derived, and serious afficionados had 2-3, hardly ever shot their carry piece.
I got a very nice but not quite new pimptastic chromed PD not too long ago for around $400. As a collector, Stars are great (as long as you stay away from the Colt-stamped collaborations which command collectible Colt prices) because not enough care to drive up prices. Sadly for the OP, but good for sellers, NIB is /maybe/ a $50 premium on things like this. I've seen factory engraved with original presentation box Star models go for plain and slightly-used prices!
I'd love some photos of a NIB version for the archives if you get the chance.