Ok, thanks for all your input. I'm still a bit unsecure here, mainly because of the medium/long neck tenon (visible in the neck cavity, but not protruding into it) and the sycamore maple top. The electronics, esp. pots, looks a bit newer. I'm very unsure about the truss rod cover etc., as I've not seen anything like it before. The seller claims there were many varieties as the guitars were produced on different factories through the years. I have seen other MIJ Burnys with three screw trcs, although without the ridges.
Anyway, with all the uncertainty of dating these guitars I don't think I have a very strong case.
Best,
Lars Erik