Depression Range-Finder

The Carter Observatory has two 'depression range-finders' in their collections - a Mark II BB and a Mark II C. They are almost certainly related to the coastal artillery fortifications in Wellington.

Does anyone know where there might be any manuals or instructions for these instruments? The Carter has no information about them and no clear understanding of how they work. The Army Museum, Alexander Turnbull and Archives NZ have all drawn a blank too.

Contact Barry Dreyer.