It's an oil/water separator for a compressor. Has a blocked inlet/outlet (unlike the Ebay oil catch tanks that have a passage straight through the unit).
It's a small size so mounting it in this cramped engine bay won't be a problem. Just need to empty it more often than the bigger "cusco" style ones.
I have one of these for my air compressor. To be quite honest, I'm not very impressed. It does catch some water, but I can still see water vapors coming out the air blower nozzle.
I'd be curious to see how it holds up, and how it works. It's meant for higher pressures. There are vanes in the thing to help spin the air around and spin any moisture or particles out before it exits the other side.
It holds up fine, and cleans some of the blowby. I had one on my car for a little while, now its just sitting in the garage. When it gets full you can drain it through the bottom. To really work tho you'd have to pipe in all the vents from the heads and crank case. Also if the cannister is too small it will create vacuum when on boost. Is that the larger model or the smaller one?