Longbox, I have one that looks like you describe, I have used it countless times, works great to adjust valves on a running engine. I have a 5/8" socket for the lock nut and the spring loaded "screwdriver" goes into the slot on the adjusting screw. The bad part about using it while the engine is running is the feeler gauge gets chewed up, but you can get the valves set just right very easily. I bought mine for my first car (a 1937 Chevrolet with a 216 engine) in 1949, and it is as good as new.
