What intrigues me is the fact that there doesn't seem to be a "1" on the scale, even in the resting position it is in appears to be a '1.5' reading. or something similar. The "V" in the upper arm makes me think it is for measuring something flat along an edge, or making sure it only hits one very small area since the whole 'arm' will swing up and change the alignment.
Still have no idea what measures start at 2 and have the aforementioned 3=7, 4=8, 5=9.
I am actually intrigued by the milling marks to either side of the reading line, as well as the fact that it looks like the scale is see-through plastic?