Well, it was most likely sold as a "wet-dry" filter and that is indeed a skimmer. Right compartment would have been filled with "bio-balls" and topped with a drip plate. IRC the Skimmer was "air powered" utilizing a lime wood air stone to foam the column. If it holds water and fits your space, you should be able to reconfigure it for your system, perhaps relocating dividers via "dremelling" Actually if it leaks, there is glue.
Good Find