Mid -Century Double Convex Magnified Viewfinder(Possible Tank Related)

This is a very interesting mid century item..in the form of two convex magnifying pieces of glass..that are laid belly to belly on a metal cradle..this in turn is held in a metal box..with top closure and handle..the top armed bottom have fixing holes..and the cones nature of the glass allows for metal rods to go through them..it measures 5.5x4x2.5 inches and is in overall lovely condition..a couple of small nibbles on the glass corners,that cannot be seen when the case is closed.The viewer sees an upside image when looking the glass..This could be a couple of things..but my gut feeling and research lead me to tank viewer..It definitely a very interesting object and a conversation opener..Registered post is available for an additional five euro.Thanks for looking please check out my other ads.