Thanks for all the replies last week.
The square hole is working fine.
I unscrewed the cables from both sides of the gear box.
We checked that the cables did move when the other ends where rotated.
Ok, so the cable is ok.
The gear or coupling box really is working since twisting one side
makes the
other turn.

What is the electrical stuff for?  There is a 4 wire connector?

I found that if I push the lower cables jacket toward the wheel hub
from the inside
the speedometer works.

So it was working for about 10 miles and now has moved and is not
working.
Any advice on which part to change?

I assume the lower cable and jacket?