1. This is included in the "circular" rules; although its outer form is "square" its operation is "circular". Unlike other circular rules however the scales are operated not directly but by turning small knurled knobs which engage with the edges of the cursor or the moveable scale as appropriate- in the top left for the cursor and top right for the scale.
2. A copy of the instructions can be found here.
3. I've also included an image of the owner's name as we have a French rule, owned by a Major (US or British rank) whose surname is the German word for "castle".

My thanks to Jim Bready for providing the images of the rule and additional information.