Thanks for pointing to the alternative, David!
Here is an extra tip. After mounting the gear shifter onto the linkage, the shift lever might still need a little fine adjustment up or down. To perform this work, the Wunderlich instructions say to loosen the upper ball joint at the linkage:
I advice to loosen the lower ball joint:
I know it does not look easier using the lower ball joint, but I found it is. Firstly there is lesser room to move the spanner around the upper area. Second, on the upper ball joint you can't see the tiny hole where the small cotter pin needs to go in. Look at the second picture, the dot left side of the ball joint, that's the hole for the cotter pin.
1. Loosen the nut on the lower ball joint with the help of a 10 mm spanner, which is bend as shown with the last picture of this post.
2. On the right side of the ball joint, just opposite the cotter pin hole there is a small recess which you can grab with a flat screw driver to lift the cotter pin carefully over.
3. Pull the cotter pin up to remove it.
4. Release the ball joint from the ball.
5. Adjust the length of the linkage by turning the ball joint. Count the turns, as a basis if further re-adjustment is necessary.
6. Clean the ball and the ball joint, and then apply some fresh waterproof grease to it.
7. Press the ball joint back onto the ball, until there is an audible 'snap' noise.
8. Put the cotter pin back to its place making sure it is seated properly, otherwise one day you may get stranded without gear shifting!
9. Tighten the nut.