It's a bolt gage and came with the empennage kit (I think). You put the head of the bolt on the top edge (I believe your picture shows it upside down) and whatever hole the end lines up with tells you the length. You should mark it with a felt pen, being sure to use the proper convention. I don't remember what the first hole is (-3 maybe?), but remember that bolt lengths are in eigth inches so the number sequence goes like this: 3,4,5,6,7,10,11,12,13,14,15,16,17,20,21,etc. The other holes are to measure diameter.