Maybe it was blocked off for some reason. I think I saw a 7-speed transmission in the parts diagrams online for Germany to comply with the speed limit laws. Maybe you had that "feature?"
On the other hand it could have been fiddled with before you bought it. we bought a former state road mowing non-ford with a similar 4 speed 2 range setup and the linkage for 3 and 4 had evidence of intentional tampering to disable those speeds. some external lube lines for the gear box were messed up too. Took a day to straighten that all out and 4th still didn't move due to non-use. I think the detent was rusted as there was no obvious mechanical issues and because suddenly with some frustrated shifter jamming at the end of the day I got it to go and it's never re-rusted in 5 years.
You may want to lift off the shift cover and see if there is anything strange in there or a missing bolt/pin?