Push the escutcheon plate inwards toward the door panel with great pressure to reveal the shank of the window riser handle. There are four sides. You want to be able to see the pin that has been shoved through the handle and window riser shaft. With the escutcheon plate still pressed firmly inward, take a pointed instrument such as probe and push one end of that pin so that it comes out the other side. When the pin has been pushed out, pull outwards on the riser handle. It should just slide off. It may take some wiggling, depending upon how long the handle has been in contact with the riser shaft. The pin is the only thing that secures the riser handle to the shaft.
Alternately, some aftermarket window risers are held in place on the shaft by a screw or Allen head plug. You'll be able to tell whether your handle is secured by a pin or a screw once you push the escutcheon plate in as far as you can. Reverse the procedure to reattach the handle.
These instructions are for a 1929-early 1931 Cabriolet, but I assume the window riser handle mounting is the same for the late 1931 Slant windshield Cabriolets?
Marshall
1929 Cabriolet