You sure you are not talking about a "tappet wrench"? Made for
adjusting valves on old flat-head engines, they are quite thin.

I'm sure. Some years ago I was out biking without tools, and needed to
adjust something. There happened to be a bike store nearby, and the
sheet metal wrenches were what I bought. I believe that they are made
thin to be as light as possible - intended to be carried along while
biking.

Dave Johnson