Renner upright action in a Schimmel and there are no eyes on the letoff regulating screw for adjustment tool. What tool do you use in this case?

A regulating screwdriver. There are some made for this purpose.

I think I finally found the appropriate tool, if not, I'll fabricate one.

Before I bought regulating tools, I made my own tool for spade-head screws by drilling a hole (about 3 mm diameter) into the end of a rod (about 5 mm diameter) and then carefully hammering the rod flat, to obtain a slotted tool. Inserting a thin flat-bladed screwdriver into the slot while hammering ensures that the slot is not hammered completely closed.

Now I use a drop-screw screwdriver, and this works fine. (Although some of the Renner spade-heads are a bit wide, and barely fit into the slot - so I still use my hand-made tool for those.)