Pocket <a href="https://en.wikipedia.org/wiki/Stereoscopy" class="extiw" title="en:Stereoscopy">stereoscope</a> from Zeiss with original test image. Unknown date, but probably from the fifties. This type of instrument was used by military to examine stereoscopic pairs of vertical aerial photographs.
<a href="//commons.wikimedia.org/wiki/User:Alvesgaspar" title="User:Alvesgaspar">Joaquim Alves Gaspar</a>
CC BY 2.5
<span class="int-own-work">Self-photographed</span>