Mask Overview: Edit
The PC 40 is a mask that was is one of the stereotypical styled masks like the Italian T 35 and French Ajax F2 that began to be produced with an educated guess from the very late 1930's up to the Second World War. It is a simple mask with the inhale thread and valve on the bottom facing slightly at a downward angle of 15 degrees. The thread is 40mm and accepts most filters. The exhale valve is located on the nose, like many other masks, and does not feature a voice diaphragm. It has the ability to be screwed off and the actual rubber valve is similar to the Belgium M51 just styled differently. The eye pieces are make out of a plastic type material that can be prone to signs of aging like colour fading and age spots. The head straps are typical, attached at 5 points to the mask, 2 at bottom sides, 2 at upper sides and 1 at the very top and use the regular buckle system to hold them in place. The rubber of the mask is very thin and easily flops around.
There is no 100% confirmed filter but there have been spottings of a yellow filter that had "PC 40" stamped on it. It has been known, when bought, with a yellow version of the filter that is very similar to the Italian T 35 filter.
There only a handful of the PC 40 ever seen or listed. Usually masks, filter, and boxes are listed occasionally but its rare to ever see a full version.
There is also a PC 35 listed on GasmaskKing.com but nothing about a PC 40
Using references and an educated guess, PC most likely stand for "civial population or protection" as it was most likely issued to civilians as the T 35 were issued to the military