According to CartWinPro, The “P” stands for the manufacturer: Polte. The whole company name is A. Polte Armaturen- und Maschinenfabrik A. -G. in Magdeburg Germany. The citation for this code is Preuss, Johannes (2002). Zahlencode-System des Heeres van 1925 bis 1940. Schwabisch Hall: Journal-Verlag Schwend GmbH. 3-935210-19-1, page 98.
The “M” supposedly stands for the Factory. The citation for that is Wolfgang, Kern. Das Tankgewehr Mauser M1918 (The anti-tank Mauser Rifle M1918). ISBN 3-933481-06-6. I’m sorry I don’t have more information for you, but I’ve attached an image of the example in CartWinPro.
