This scope was made by Carl Zeiss in Jena. The Zielvier 4×81 model characteristics were used as a standard for the “ZF39” denomination. Following its meter dial and serial number, this scope was probably used by the Reichswehr in the late 20’s or early 30’s.
Below you can see two pictures both showing German snipers using K98k mounted with Zeiss Zielvier scopes on turret mount.
Your example carries the serial number of 20863. I have an example with a serial number within 1,000 of yours (218xx). The front bell is slightly different and mine has a “lug” just ahead of the turret. Is mine of the same vintage as your example? Would you have a guess of the value?