What are the differences except for the form of the rewind knob?
Canon's Camera Museum pages (their error on the finder aside) do not show other differences.
For the 1.7 eqivalents they state: "The “G” in the camera designation stood for “Grade Up” which referred to the quality improvement."
I have a QL19 GIII that has a battery check light here in front of me. The QL19 next to it does not have a light. Both have the PC cover and neither have the cocking indicator.
The rewind knob is different, I'll report back with more info on internal difference once I can get them both opened up.