[Fullraceflathead]

Yes the slot area is very worn. Mine was almost the same. I made my pin oversize in diameter and a little longer so it went into the unworn area of the slot.
The Pin is available but to really fix the slop issues I would have the worn areas of the slot welded up. It doesn't take much of a weld bead to build up the worn areas.
Your AA is the later version with the threaded shifter cap, that's a good thing. The caps are available new old stock. They don't wear much though.
Be very careful that pin doesn't drop into the transmission. If it does you'll have to either use a magnet to retrieve it or remove the PTO cover and get it out, unless your lucky enough to have it pass out the drain plug hole while draining the oil.
The pin was missing in mine and some one used a stove bolt in it's place. It had so much slop I could move the shifter 7" or more in all directions even when it was in gear.