Buckles made for the farm equipment company Hesston (now owned by the AGCO Corporation in Georgia) have become highly collectible. The first buckle was made in 1974 as a form of advertising. They became associated with the Rodeo Cowboy Association and issued many NFR buckles. Other classic cowboy themes were tried in later years with usually one buckle per year.
Several companies have produced the buckles over the years including Ruebrow Manufacturing, Lewis Buckle Co., A&E Die Casting, Cast Products, Gary Gist, Award Design Metal, and most recently Montana Silversmiths.
The most collectible are the 1974 and 1975 but you need to be careful of fakes.
I carry only the adult size buckles - there are also youth buckles that started in 1983. And there are silver/gold higher end versions for some years but I don't carry them.