Union kind evolved in a tangled web from Colton and Hickox (1891), later Colton Manufacturing Co. by 1894. By 1901 Colton was listed as gun manufacturers in the Toledo directories. Colton was manufacturing guns for Sears & Roebuck which appear in the 1900 and 1901 Sears catalogs. On September 8, 1902 the Union Firearms Co. was incorporated. Sears needing a supply of cheaper repeating shotguns than the Winchesters or Marlins, had a plan to acquire the remains of Spencer, from Francis Bannermann who had been manufacturing the Spencer pump, and move the equipment to Toledo. This apparently went wrong, and Colton and his people (including Charles Lefever, son of Uncle Dan) went ahead on their own to manufacture a pump. Union made a slightly different version of the pump for Sears. Union was also manufacturing a double with ejectors by 1903. Both this and the pump were in Sears 1903 catalog. Their last listing in the Toledo city directory was 1913.
By Union Fire Arms Company's 1905 catalogue they were making a single trigger double with a Lefever patent single trigger, steel, twist or Damascus barrels at $25, $26, or $27. Engraving $3.50 extra, fancy figured stock $3.50 extra, recoil pad $3.50 extra, and ejectors $10 extra. Union's Model 23 was their finely finished gun at $60 with Laminated or Damacus barrels, ejectors $10 extra. The Model 22 was their plain double with steel, twist or Damscus barrels at $18, $19, or $21, ejector $10 extra. Model 24 was the pump gun at $19 with steel barrel, $20 with Laminated barrel, and $22 with Damascus barrel. They also offered two hammer single barrels. By the 1911 catalog they had added a Model 25 Peerless pump gun, and a Model 25A which was a trap gun. I've condensed this from Joseph T. Vorisek’s "Shotgun Research Newsletter" volume three, issue one.
The only Union paper I have is a little catalog reprint by the late Dr. Wm. Paul Smith. Here are scans of the double gun pages --
Note the cheapest gun the Model 21 cocks by movement of the top-lever, used decades later on the Savage Model 420 and Model 430 over/under.
I believe there was also an article on Colton/Union by Cdr. Roy Gunther in
The Gun Report, but I don't have the reference.