U.S. Navy Mark V mixed gas diving helmet used from 1916 up to the 1980s.
https://s3-us-west-1.amazonaws.com/h...k_fullsize.jpg
Previously:
https://www.homemadetools.net/forum/...435#post181776
https://www.homemadetools.net/forum/...570#post174132
https://www.homemadetools.net/forum/...110#post172312
https://www.homemadetools.net/forum/...635#post136442
https://www.homemadetools.net/forum/...774#post133170