Looks a lot like the Belgian M3 version of the 50.63, but with a ~406mm bipod-cut barrel (M3s had a non-bipod-cut 436mm barrel), bipod-cut handguards, and a Type 3 upper receiver (M3s had a Type 1.5) and Type 1 lower receiver (M3s used a Type 2).
I say this because of the presence of the carry handle and a standard charging handle, as well as the hiduminium lower receiver, which are not standard features on the typical 50.63 but certainly are for the M3 (of course, this is not an M3, it is just configured a lot like one). You likely have other hiduminium parts as well (trigger guard, pistol grip nut, pistol grip stud, stock bolt; I can't recall if the hinge blocks for the stock were ever made of hiduminium), which can be verified with a magnet. The pistol grip is likely serialed. The rear sight is the fixed type typically found on the shorter-barreled folding-butt rifles (and also found on Dutch-contract fixed-butt rifles). It has the narrow-foot bolt hold-open device and the version of the combo device with the flats near the vents, which is the scarcer type, it seems, as well as a vertical frame-lock lever.
I'd say it is a somewhat non-standard lightweight FN 50.63. I don't think FN ever made that barrel length as a production item, but I could be wrong, but if I'm right it means someone cut it down. I'd have a gunsmith make sure that the gas port is the right diametre for that barrel length just in case it was cut down and the person who did it neglected to open up the port (or did it improperly), or you will have reliability issues. It's a very nice piece. The lightweight version is rather desirable and nowhere near as common as the standard versions with steel lowers.