The Czechoslovakian OM10 is a common surplus mask and is usually sold for around $20-40 online.
The mask is inspired by the US M17, but has many clear differences. The exhale system is different, having a metal housing with the exhale pointing directly outwards, instead of the M17 style exhale, where the valve is covered by rubber and forced out through the bottom of the housing. The filter openings are larger and threaded. The head harness varies as the OM10 head harness is made of a thicker grey elastic fabric with a rubber head pad and large rubber pull tabs.
The M17’s outserts have been found to fit with OM10, but are noticeably smaller, and the filters are not interchangeable between the masks, due to the different inhale sizes, and cap attachments. The OM10 mask was used by both Czechoslovakia, East Germany, and remains in use today by the Czech Republic.
An upgraded model entered production in the late 1970s, designated the OM10M, which changed the head harness, and the voice diaphragm/exhale system. It was changed to a large grey plastic piece, with a long “M1” style drinking tube wrapped around the assembly with a rubber flap folding over it