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Gord, this is a Davis Instruments Prism Level which fits their Mark 12, 15, 20 and 25 sextants. I have a one I got with an old mk 20 sextant that needed some work. It's purpose is to determine when the sextant is vertical without the need to "swing the arc". When properly adjusted the horizon as seen thru the prism and the horizon glass will line up. If you find it intruding on the view when bringing bodies down you can flip it out of the way and then filp it back into view when checking the vertical.

You might find a manual for it on the internet - I didn't have any luck yet. I thought I might have one for this model but couldn't find it. I have the repair parts for the mk20 and an old mk12 (in worse shape) but no manuals stored away with them. Some cold winter's day I'll get started on them.

I did find this old ad for a Davis mk II version of the prism level - sold some time ago. It was made to use on circular horizon mirrors. An image of a page in the manual shows how it's used but not how the adjustments are made when setting it up:  http://landandseacollection.com/id745.html

Robert Pearson