I have this speed wrench with a fixed socket slotted screwdriver.  The screwdriver is engaged by pushing the shield back to engage the slotted driver and it is just over 20 inches long.  The only marking on it is "Piohi".  Anyone know anything about it?

Very deep screw starter ?

Pre 1963 6 cylinder Chevrolet engines had slotted screws for the valve cover, tappet side cover, timing cover and oil pan. They were commonly called stove bolts. I have the same socket in the Proto brand. The sleeve around the slotted screwdriver bit kept the bit centered on the bolt. I spent 40 years studying, repairing, rebuilding and hot rodding 6 cylinder Chevy 235 and 261 engines. I got real sick 9 years ago and sold 22 engines and all the hot rod speed parts and only kept the tools for working on them.

EvilDr235

Yep. Here are some Snap-on and a Plomb socket for that purpose.
-Don

Maybe to hand prime oil pumps on certain engines?

My Proto socket looks just like the the picture of the Plomb socket in d42jeep's picture. It is a useful tool when you have to remove a lot of slotted bolts in a hurry.

EvilDr235

Thanks all, now I know.