I'm almost 100% certain it's a rebox of the Esci kit. I recently finished the Kettenkrad after a hiatus of 35 years.
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The decals were unusable after all these years, so I ordered them via Carson, the distributor for Italeri in Germany. Thereby I noticed Italeri fixed at least some details on their kit, at least on the dashboard. So maybe they fixed some things for the Zündapp, too? But I would not count on it.
Off the top of my head, Esci released quite a few kits in 1:9 at the time (early/mid 70ies): BMW R75 single and with sidecar, Zündapp KS 750 single and with sidecar, the Kettenkrad, Harley Davidson WLA-45, Triumph 3HW and the VW Kübelwagen.
There were also a few figures available: German regular soldier and paratrooper, and a British paratrooper with helmet and Denison smock.