The antigrav is a bedroom-sized cube of smooth sheet metal walls encasing complicated metallic machinery inside. When switched on, it generates an anti-gravitational field enough to lift a stone building like the Cathedral of Notre Dame to an arbitrary height in the air.
This machine consumes roughly as much coal as a early 20th century steam train locomotive, and works in a similar way on the surface, spewing as much smoke and steam during it's operation as the steam train locomotive, although it's principle of operation is fundamentally different, a truly unique invention. If the steam locomotive train stops upon ceasing coal, and just sits on the railroad, the antigrav machine stops and starts falling down, with everyone on it.
Hence any airship, or flying building which has at least one antigrav machine, must make sure that there is enough coal for it's operation, and also that this machine is well protected against external bombs and missiles.
Having such antigrav machines, what kind of military uses could it be used for? I was thinking of maybe huge flying aircraft carriers, or battle ships, but in the sky. Perhaps even flying castles with lots of howitzers. Multiple antigrav machines can be placed at various parts of the structure.
In this alternate history, the technological level is similar to OTL 1950s, excepting the presence of this new invention. There are planes and helicopters, and there are no nuclear bombs in this world.
What would be possible military uses for antigrav machines in this world, and what would be their impacts on warfare? Keep in mind that the military men of this word are not always exactly sane, having practically unlimited budget, and would have no qualms about building big, ugly, and fearsome, but not necessarily practical and effective monstrosities.