The Czech fatra F-M-1a was manufactured and used pre-WWII and for some period of time near the beginning of WWII (exact dates unknown).
It uses a steel frontpiece for the filter input and exhale valve.
It uses tissot tubes, but has no orinasal cup.
German Wehrmacht lens inserts work with the mask perfectly- whether this was intended or not is unknown.
The mask was also used by the Kingdom of Yugoslavia, likely by contract from Czechoslovakia as part of the Little Entente.
The mask uses a six-point elastic harness with a circular leather pad. The exhale valve is protected by a metal housing, which can be screwed off for maintenance and cleaning.
This mask is fairly hard to find, especially in good condition.
Carriers for the mask vary, probably depending on time period, from cardboard tubes to sturdy metal canisters similar to the Romanian carriers of the time period. The Czech Fatra was later made in many different variants, the FM-1b, FM-1, FM-3a, FM-3b, FM-3c, FM-3, FM-4b, and FM-4.
The FM-1a is the only FM-1 variant to use a metal facepiece. The FM-1 and FM-1b both use bakelite facepieces, the difference between the two models being that the FM-1 had an angled filter port whereas the FM-1b used a straight filter port (which was simpler to manufacture but caused the mask to be unwieldy with heavier canisters).
The FM-3 is a much simpler and cheaper model, with a different harness mounting system that was much easier to manufacture but also much more prone to malfunction or breakage. The FM-3 variants are all just slightly different from each other in terms of material used for the front piece and the mask facepiece itself.
The FM-4 and its variants were designed and made for children.