Intercom motorcycle helmets from the 1950s.
https://s3-us-west-1.amazonaws.com/h...e_fullsize.jpg
Previously:
https://www.homemadetools.net/forum/...722#post216491
https://www.homemadetools.net/forum/...449#post219272
https://www.homemadetools.net/forum/...258#post172828
https://www.homemadetools.net/forum/...869#post171363
https://www.homemadetools.net/forum/...062#post125604