Looking at the 1st photo with the ruler on it:
a scale on your rule of 42 is equal to: -3 ½ inches or 8.8 centimeters, or,
36 = 7.6 cm ….. 33 = 7 cm …. 24 = 5 cm …. 15 = 3 cm
Which don’t make sense to me therefore I will assume you have a “Barnstick” type instrument that could be old as dirt.
Now for those who don’t know what a “Barnstick” is, ….. it is a fairly precise instrument made by farmers of old when they needed to build themselves a barn. Since they didn’t have a tape measure or a rule they would whittle themselves out a pretty new “stick” of wood about ¾ to an inch square and yea long (between 3ft & 6ft, I guess, it didn’t matter). And they would use that Barnstick to measure all the dimensions of their new barn. Like 20 “sticks” long, 10 “sticks” wide and 12 ½ “sticks” high. And they would cut little “notches” all over that “stick” for other dimensions, and thus they could measure how wide or long the boards n’ beams n’ rafters, etc., needed to be cut for building their barn.
Your instrument could be so old that whoever made it chose the “measurement scale” to fit the size of the physical instrument, …… aka: the Barnstick.
Whatta I know, ….. it makes for a cute story. Cheers