Metallwarenfabrik Wissner AG was established by Heinrich Wissner in the town of Zella-Mehlis in Thuringia in central Germany. Heinrich Wissner seems to have started the business in 1882, but it appears that it was not formally registered as an Aktien-Gesellschaft (AG) until 1892.
Zella-Mehlis lies slap-bang-in-the-middle of two of the main 'poles' of the German bicycle industry - Schweinfurt, home of Fichtel & Sachs, and Chemnitz, home of Diamant. Inevitably, the town hosted a number of cycle component manufacturers. Wissner was most famous for manufacturing bicycle bells, but also seems to have produced bottom bracket parts, brakes and derailleurs. The company appears to have been successful, growing to some size and operating 5 different plants in the locality. During the Second World War it is possible that it produced arms and munitions.
Wissner used a distinctive 'clover leaf' logo.
After the Second World War Zehla-Mehlis was in the Soviet zone (soon to be the DDR) and, in 1946, the factories were expropriated by the Soviet authorities. Some were demolished, with the machinery sent to the USSR as war reparations. The remainder of the company was nationalised, and, in 1948, became part of VEB Meteor, a manufacturer of numerous different engineered items, including cycle parts. In 1956 the industry was rationalised again and VEB Meteor was integrated into VEB Elfa Elsterwerda.
In 1992, following the reunification of Germany, a company called Meteor Umformtechnik GmbH was established in Zella-Mehlis manufacturing precision stamped parts. This company, which is in business today, can, arguably, trace its roots back to the Wissner concern.
In terms of derailleurs, I am not clear if the 'clover leaf' logo was used after 1946. I am also not clear when the derailleurs were produced. The only instruction leaflet that I have is undated and in Italian. All the Wissner derailleurs that I have acquired over the years have been found in either Italy or the Czech Republic (none in Germany). Some have clover leaf logos, some do not. My guess, based on very little, is that the derailleurs may date from the 1940s and 1950s.