Looks quite a lot like an American Walter, but without the front body panels it could be almost anything. Walter built a line of very heavy-duty 4X4 for airport use, but then vanished in or around the 1980s (?)
this is definitely a Walter Sides built engine. Indeed the bonnet is partly removed But Luxembourg Airport had about 3 or 4 Walters including one with 2 roof monitors just behind the cab
Have a look at commons wikimedia where there are two superb shots of this very interesting Walter as well as some more excellent pictures of Luxembourg fire trucks.
Added by Rob Johnson on 05 January 2013.