Don't know how, but I managed to delete the post that went with the picture.
There is a roll pin that fits around the existing pin in the shifter. The one in the photo was a well worn top so a thick roll pin was used. I have used thinner roll pins on other covers to prevent having to file away excessive amounts of material from the cover.

You will have to file the notch down as well as side to side to make sure the ball part of the shifter fits into the cup part of the cover.

Not a real difficult undertaking. I was able to get the cover off my 140 without removing the gearbox. Don't know if there is as much room on a 122 to do the same though.
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'60 544, '68 220S, '70 145S, '72 144E, '86 745T