Permeable gear with the M3-IXA1-IVA1 Diaphragm gas mask. Permeable gear was used as early as WW1 and remained standard into the 1950s. My specific setup uses the second pattern “special” HBT uniform
Inside the PMK-1 and PMK-3 Gas Masks (M-80 - MB-2) by DrJorus on DeviantArt