The AUFTRAGS № 286828 is for 1938.
Mercedes never sold a car without a finished body.
Even if the body was ready in 1939, the AUFTRAGS number should have numbers, which are relevant for 1939 starting from 288xxx.
In 1940/41, these AUFTRAGS numbers started from 301xxx.
In 1941. AUFTRAGS 321xxx - 343xxx