The PM12M is a screened pentode for RF amplification. It is one of the early valves made with the anode as top cap connection, later types placed the control grid connection to the top cap for even greater separation of input and output circuits.
The PM12V variable μ pentode was released in 1932, and was replaced by the PM12M in 1933. We also have a version of the PM12M with a clear envelope.
PM stands for Philips Mullard, the names of the companies.
The metalised classic envelope is 43 mm in diameter and, excluding the B4 base pins, is 110 mm tall.
References: Datasheet & 1040. Type PM12M was first introduced in 1933.