I picked up this curious instrument, in its original steel case, at a yard sale several summers ago. I had no idea what it was, but it looked cool, the price was right and the case would fit on the back of the motorcycle so I didn't have to go back for it. Also, if nothing else, it had a Federal dial indicator on it that might be useful in another application.
The 1929 patent drawing below gives some idea of how it was used. The device was hung on a strand of wire, and the operator pulled a lever through a pre-determined distance deflecting the strand upwards by a standard amount, and then took a reading from the dial.
The instrument I picked up looks like a "B Strand Dynamometer" used by Bell and AT&T in the 1970's. I've uploaded a manual I found online for those with more curiosity about its function.