The "mystery item" hass arrived - unfortunately badly damaged due to poor packing, but I would nevertheless be interested to learn more about it.
The larger (let me call it "bottom") part has two (identical) flat sides - a "standing ring" and a concave "inner surface" - looking a bit like a doughnut:
(click on image for larger view)
The smaller (let me call it "top") part is domed with a flat base:
It does have some magnifying effect - but it's too thick and has rounded edges, which would make it a badly designed magnifying lens.
Now the interesting fact is that the radius of curvature is identical for both parts - if put together differently (as by the seller)
the top part will rotate smoothly: I assume this is designed to allow leveling of e.g. a small measuring instrument on a slanting surface. I am not convinced, however, that it has been designed (specifically) for use on a ship: to compensate a ship's movement, this adjustment would have to take place fast and "automatically" - which this item will not do (too much friction to work on gravity alone).
Does anyone have further information on such items - or an alternative explanation?