It is a steamer juicer and new stainless steel ones are for sale for about $180. This one is aluminum, but it seems to have all the required components.
The whole berries are put into the top, water goes into the bottom, and the juice goes into the middle part and gets drained off via the tube, which has clamps to keep one end on the pot, and pinch off the other end.

Bottom:

It wouldn't work on a flat glass stovetop because the heat would get trapped by the ridges and damage the cooktop.
Top, where the fruit goes:

Middle - the steam comes through the holes, and the hose on the side decants the juice.

So I guess I'd better keep it for awhile until I find someone who wants it, or because my wild chokecherries are especially luscious this year, maybe I'll make some juice with it, and work up to jelly. Here is the recipe over at Kitchentales: Chokecherry Jelly Recipe