https://discussions.virtualdr.com/im.../2014/02/8.jpg
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http://dailypicksandflicks.com/wp-co...ump-1051-7.jpg
EDIT- I did a little research on this one and confirmed what I thought it was..
http://butdoesitfloat.com/For-the-wo...ulating-it-allQuote:
Acoustic listening devices developed for the Dutch army as part of air defense
systems research between World Wars 1 and 2.