Cannot figure out how to remove shower handle

I have been having problems with the hot water knob of my shower, whereby the water pressure gets very low, especially after the knob is turned off tight. I am then able to restore the pressure by turning the knob all the way out (open), but am afraid that eventually this "fix" will stop working. Therefore, I am trying to disassemble the knob so that I can hopefully find and correct the problem. However, I cannot for the life of me figure out how to remove it. There is absolutely no screw or removable cap anywhere to be found (see photos). I believe it may be a Kohler, but am not 100% sure. It was already there when I bought the house in 1994.