I did a little searching and found out, as usual, more than most people want to know. Here is some edited info from megaone.com, who shows some old Yellowstone stereo images on their site.

At one time, Underwood and Underwood was the largest publisher of stereoviews in the world, producing 10 million views a year. The company was founded by two brothers, Elmer and Bert Underwood. The brothers developed a selling system of thorough canvassing using college students. They distributed stereographs for some other companies, and by 1887, they outgrew their original office in Ottawa, Kansas, and moved to New York City; By 1897 the company had a number of full-time staff and free lance photographers, and were publishing twenty-five thousand stereographs a day by 1901. The firm still canvassed and sold its own stereographs. Around 1900 Underwood & Underwood introduced boxed sets, with specific themes such as education and religion, and travel sets depicting popular tourist areas of the world. By 1910 Underwood & Underwood had entered the field of news photography. Due to this expansion stereograph production was reduced until the early years of World War I. Altogether Underwood & Underwood produced between 30 000 and 40 000 stereographic titles. In 1920 stereograph production was discontinued and Underwood & Underwood sold its stereographic stock and rights to the Keystone View Company. The negatives passed to Keystone which issued them with a "V" prefix.

The Keystone View Company was an early competitor to U&U, founded in 1892 by B. L. Singley in Meadville, Pennsylvania. This might not sound like an auspicious location, far from the photographic centers of New York, Boston, and Philadelphia, and originally the operation did indeed have a "backwoods," quality, turning out distinctly second-rate stereoviews and barely making the founder a living. But it rapidly improved and eventually became not only the world's largest but also the best view company. In the formative days Singley took all the images; later there were dozens of staff photographers, and was smart enough in the early 1900s, when stereoviewing was declining and other companies were in trouble, Keystone bought their stocks and incorporated them into their own holdings. Notable amongst many such purchases were the huge and varied inventories of B. W. Kilburn, the H. C. White Company, Underwood & Underwood.

By the 1920s the Keystone Company was the sole surviving major producer of conventional card-mounted views anywhere in the world. They had offices in London, Paris, Sidney, Capetown, Rio de Janeiro and Tokyo. Part of this prominence was derived from the sale of World War I stereograph sets, as Keystone was one of the only publishers to secure permission to photograph battlefields and military operations. Unfortunately, permission was not granted until the end of 1918, just before Armistice, so many of the photographs depict scenes taken after the end of the war.