The history of Union (and ESKA and Favorit) may go something like this. In the late 19th century the hub of the Czech bicycle industry formed in the city of Cheb. The Cheb museum website indicates that a company called Premier may have been making bicycles in Cheb from around 1880. There is a claim on Velobase that Favorit celebrated their 100th anniversary in 1974 indicating that Favorit, or possibly its forebear - Premier, was established in 1874. Cheb is very close to the Czech/German border, and is a mere stone’s throw from Chemnitz, the epicentre of the early German bicycle industry and home of Diamant. Cheb is also one of the main cities of what was once called the Sudetenland, where in the early 20th century, despite being part of Czechoslovakia, German was the dominant language and, arguably, the dominant culture. This seemingly simple sentence describes complexities which have led to untold suffering, death and destruction - you just can’t beat a spot of nationalism for fueling vicious idiocy.
In 1911 two employees of Premier, called Swetlik and Kastrup, formed a new bicycle manufacturing company in Cheb called Elite. They soon changed the name to ESKA (made up of the phonetic versions of the first letters of their surnames ES and KA).
In 1922 ESKA opened a factory in Rokycany, near Plzen, home of the famous beer. This plant originally branded its bicycles as Tripol.
At the end of the Second World War the government of Czechoslovakia nationalised all bicycle factories that had German investors - and this included both Premier and ESKA, which were possibly merged into one company called ESKA. This merged company may also have used the Union brand. In 1953 the operation in Rokycany may have been separated from the main ESKA company and then adopted the Favorit brand. It is possible that at this point the Union brand fell by the wayside.
In terms of derailleurs, I only know of one Union design, the model in this collection.