I'm a bit confused. Photos have serial #1566154. This is 1934. You write number 4017860 which last 1960s.
Insert looks to be handmade later. Original would have some stamped elements for indexing and film location, and be black anodized or painted.
You use the 6x4.5 mask by moving a number from one window to the next before moving to the next number in the first window, etc. For full pleasure of120 film, try 6x9 (remove mask) at first.
Lens designs were changing fast in the 1930s. 4.5, 3.8, 3.5, etc. It probably comes up as rare because not many cameras from the first half of the 1930s are still around.
If the rangefinder is either off or dark, you can still do scale focusing and get good results. A nice 'pure' camera when Zeiss was making some of the best.