This came with a few photographic things I'd been given. The owner didn't know what it was or what it was for. The only text on it is "CENE" and "Germany".
It's "CENEI" not CENE. This is a mark of Carl Neithold, see camera-wiki: http://camera-wiki.org/wiki/Neithold a manufacturer of many kinds of accessories. Googling shows there is a similar one on ebay at the moment: https://www.ebay.com/itm/155870539269 and the assembled picture makes me tend to agree with Matt that it's a slide viewer.
It's "CENEI" not CENE. This is a mark of Carl Neithold, see camera-wiki: http://camera-wiki.org/wiki/Neithold a manufacturer of many kinds of accessories. Googling shows there is a similar one on ebay at the moment: https://www.ebay.com/itm/155870539269 and the assembled picture makes me tend to agree with Matt that it's a slide viewer.