Motorcycles had been DKW's mainstay since the early 1920s, to the point that by 1939 DKW was the largest motorcycle manufacturer in the world. In the aftermath of the Second World War it was again motorcycles, especially lightweight, cheap motorcycles like the RT125, that helped the company re-establish itself in West Germany. However, as the war devastated economies of Europe began to recover during the early 1950s, motorcycle sales began an inevitable decline. Year on year, DKW's motorcycle arm began losing money and in 1958, when Mercedes-Benz took a controlling stake in Auto-Union AG, one of their first actions was to divest themselves of DKW's motorcycle division.
In a mirror of the foundation of Auto-Union in 1932, DKW's motorcycle division merged with two other struggling motorcycle companies, Victoria and Express, to establish the Zweirad Union, or Bicycle Union in English. Each of the three companies retained its own brand name and model range, but they pooled their development and factory resources. The new company was headquartered in Nuremberg. There was one exception to the merger- the DKW RT175 motorcycle continued to be built at DKW's Ingolstadt factory.
With the market for big motorcycles drying up, Zweirad Union focused on lightweight motorcycles and mopeds, like the DKW Hummel, which had been a popular seller for a dozen or so years. Victoria and Express had generally used proprietary engines from manufacturers such as JLO, Sachs and JAP. ZU gave them access to DKW's trusty two-stroke engine, sometimes rebadged, sometimes not. There was a great deal of confusion amongst ZU's product offering as each company continued to sell its own products. Some bike however were cross branded and rebadged, some were even sold under the ZU banner. A single machine might have four model numbers, depending solely on the branding.
Advertising was bright and cheerful and targeted at the youth market. Nonetheless, sales were never particularly strong and in 1967, ZU was bought out by another small motorcycle company, Hercules. The Express and Victoria brands were retired shortly thereafter. The DKW name continued, but only as a export brand name for what were effectively Hercules motorcycles.
Engine manufacturer Fichtel and Sachs bought into Hercules in 1963 so the takeover by Hercules spelt the end of the DKW two-stroke. From then on Sachs engines powered ZU machines (although the engines were in fact very similar). Hercules continued making bikes right through into the 1980s. Sachs continue to manufacture engines today and have recently released a new range of motorcycles and scooters. http://www.zf.com/brands/content/en/sachs/homepage_sachs/homepage.jsp
This Zweirad Union user manual dates from 1965 when DKW, Victoria and Express were still brands. The manual is very generic and basically covers all their various models.