This is a Bell System pull finder that I used as an Outside Plant Engineer to determine corners and find anchor locations. The device has a pivot in the middle. Rotating on the pivot are one main arm with threads on one end and two larger arms with pointers.
The device is used as follows: The solid arm is screwed into the pole at chest level on the opposite side of the corner. The other two arms are then positioned so that they point to the poles on either side of the corner pole. The two arms have sights on them to assist in ensuring the direction they are pointing. The left arm has a pointer that rides over a scale on the right arm which reads "PULL ON CORNER - FEET". In the picture the pull is measured as 20'. Then without moving the left arm, the right arm is rotated to the left until the notch (on the left side of the right arm scale) lines up with 20' on the scale on the left arm. That makes the right arm point in the direction of where the anchor will go.
There is another tool, a thumb wheel, that I used to determine the size of the guy and anchor based on the pull, that I got from the pull finder, and the distance to the anchor from the pole. I don't think that I have one.
The pull finder also has a measuring device used to determine the size of existing guy strands and anchor rods. It has the standard sized guys of 6.6M, 10M and 25M, and the standard sized anchor rods of 5/8" and 1". I'm not sure what the M stands for ... maybe millimeters?