This horn antenna is for 24 GHz and has a gain of 20 dB it was made in 1982. The quality of the construction is evident from the filler that has been used to soften appearance of the joints between the sheets forming the flare. Note also the re-inforcing of the outer edge of the flare.
The other quality feature is the careful matching of the flare to the waveguide. A square flange is used on this horn and the would be screwed to the equipment it was working with.