How do I get this shower handle off?

[BettyLou]

We bought a 1973 ranch house and have been attacking the rooms one at a time. One of the showers is missing the handle, but we can control the water with the stem, not ideal, but I cannot find a faucet handle that will work. The other shower needs to be gutted, but the handle looks like it could potentially work as a replacement. The trouble is, I cannot figure out how to get it off. There isn’t a face that can pop up, and when I looked underneath there is a hole, but it’s too small for my hex wrench. I would appreciate any help!

 

[hj]

Then you need a smaller hex wrench, because that is where the screw is.