Calculateur de Parallaxe / Parallax Berekenaar (Parallax Calculator)

We frankly have no idea exactly in which specific application this Parallax Calculator was used (aeronautical or astronomical or ?).  This large device measures ~13 inches square and has 3 clear disks and 1 clear arm that rotate around a center axis affixed to a metal-rimmed wooden base board with what appears to be measurements of deflection.  The disks may be secured by turning the 4 metal knobs.  It is labeled in French and Dutch. 

A cool piece regardless of its intended application.  We guestimate that it is from 1950-60s given its design and materials of construction.  Knowledgeable visitors are invited to email us with information about this curious item.