There are three green numbers on my Canonet QL19’s aperture scale:
The numbers are 28, 20, and 14. What are those?
They are flash guide numbers (in meters, for ISO 100). This is for the QL17, but see Cameraquest, and check out the manual here (page 17). Apparently many rangefinder (and point and shoot) cameras of the day used the "Flashmatic" system, where the camera automatically selects an aperture to match your focus distance and flash power. So, set the ring to the guide number of your (manual) flash.
If you're not familiar with using guide numbers, see How can I calculate the effect of non-TTL flash on exposure? The Flashmatic system seems like a pretty neat way to simplify this without needing flash automation, although basically rendered obsolete with TTL metering in SLRs.