I can confirm the information provided by Snakedoc, that's a very interesting find.

The tool was used first vertically to determine distance using known or estimated target height (At mast top, superstructure, funnel, etc) and then rotated 90º and used horizontally to determine angle on the bow, i.e. relative inclination to the observer's line of sight, which in turn allowed calculating the target heading. The periscopes also had some more interesting tools, like the "fixed line in space" and a secondary ocular where you would see s aseries of adjustable scales to construct the firing solution.

Is the graticle still in the lense, or does the part you purchased only have the double prism? I'd kill to see how the reticle looked like, there are no trustworthy images of that anywhere to my knowledge :-(