Dutch Model K
The Dutch Model K was made of dark green rubber with two large round eyepieces and an exhale valve jutting out to the side. This example was made by Hevea in 1953.
Similar masks in my collection: Italian Tipo Super 53
Manufacturer and model stamps on cheek "T.1 18-08 [H] K-53"
Stamp inside mask
Stamp inside mask "3."