It is a Police Call Box; "M" and "P" would be the initials of the department or Providence etc... Blue color is indicative of Law Enforcement call boxes.

432px-StMartinsLeGrand-PoliceCallBox.jpg Police_box,_London_EC3_-_geograph_org_uk_-_2836010.jpg

Buying: Polarine / Red Crown Gasoline Globes and Signs, Early Chevrolet & United Motors Signs, and 1910's through 1940's Gas & Oil Signs.