USSR, Three bolt equipment, 1951years.
Three-bolt diving equipment, "three-bolt" - equipment for safe diving underwater, classic diving suit. Copper (made of sheet red copper) diving helmet with screwing window (viewing window), attached to manish by three bolts (hence the name). Grass-safety slide valves are located on helmet. The thickness of the windows is 12 mm. Helmet life is 9 years.
It is used for emergency rescue, ship lifting and other works at depths up to 60 m.
Helmet made of copper and brass