It's an infra-red monocular powered by a 'Zamboni Pile' (a very low-current high voltage battery) which is connected when you press the button on the case. (The batteries are not replaceable as the entire equipment is sealed at the time of manufacture - and they're all over 75 years old by now - they were dead in the 1960-70s.)
They were used for navigation, etc., and the optics give an inverted image.
Clive Elliot produces a series of articles on night vision equipment (Tabby, etc.) and you can find it here
https://hmvf.co.uk/topic/20066-tabby...ghtvision-1-4/
"Tabby" was a vehicle-mounted night-driving equipment, but this monocular is also described in some detail.
Best regards,
Chris.