I think the one on the left is what I used. I didn't go out and buy one because I had an assortment and just found one that would fit pretty snug. Did not have my manuals at the time.
I got a 6mm x 24 roll pin from the hardware store.
It was a bear to get in since the hole in the tractor is higher than the hole in the shaft. Had to put some more chamfer on the split pin with a grinder and first start it prying with a screw driver then I could pound it in with a punch.
Everything works good now.
Excited to try out the 4x4 in the snow this winter...