I've just checked a Kowa brochure on the TSN-550 series, but can find no mention of the housing material. Also drew a blank on the German website. Externally they are very similar to the 880 series, so I would expect magnesium alloy but there are anomalies. The Genesis 22 and 44 binoculars have alloy housings, whereas the Genesis 33 is composite.
I had a brief look through a 553 at Photokina a couple of years ago and, while it is optically excellent, it's very expensive for such a small scope. The permanently attached zoom eyepiece has a rather narrow FOV and is by no means comparable to the TE-11WZ 25-60x wide-angle on the 880 series. Personally, I would have preferred it with a fixed focal length wide angle eyepiece giving around 25x magnification.
John