The slots on the clamp arm... how did you make the slot from the shaft hole longitudinally towards the pin end???
The only way I can think of is that you slipped a fine-bladed jeweler's saw into the hole and went at it! If so, you did a very nice job of it, it looks machine cut.
I think Swede was referring to your dividing head arm as shown in the updated photo. I noticed that slot too and am curious how you did it. Very nice work by the way.