Your thumb screw is slightly different than mine, there were probably several versions of this. But check the pictures to see if you have a set screw in the side of the shaft, removing the set screw shown allows me to flip the insert to 3/8 and 1/4 fittings.
View attachment 213957 View attachment 213958
The knob at the bottom of your last picture is the shutter release lock.
The spring lever at the top of your 3rd picture will pivots a lever onto the shaft of the thumb screw, which prevents the thumb screw from turning CCW. This allows you to remove the grip from a tripod if it is really stuck tight. If your thumbscrew does not have a set screw, try using the lever to prevent the thumbscrew from turning, and see if you can unscrew it into 2 pieces, and see if there is a invertible 1/4-3/8 insert.