Two patents were filed by Edward Bullard specifically depicting the Bullard masks's facepiece, both are shown here. Two exhale valves were also patented, and those are shown below.

It is believed that the second patent shown here (May 7th, 1940) describes the later Bullard masks, as the exhale valve depicted closer resembles the plastic covers found on later models.

The exhale valve shown above describes the later plastic exhale valve system.

It is unknown which masks, if any, use the valve/cover combo shown above.

gasmaskbunker.com (c) Moulage, 2017. Drawings (c) Canis-Infernalis.