I have a bunch of these things, and I think every one of them is broken. That's not the fault of the camera, I bought them cheap to learn how to work on them and then never got around to it. Or started it and got stuck. (I have at least one in pieces in a bag). Some day...
There are a lot of different variations, but the basics are the same. Canon had made high-end 35mm rangefinders for decades, many of them with interchangable lenses. But in 1961 they decided to go down-market with a mid-tier, fixed-lens rangefinder, which they called the Canonet. According to a book, "How Canon Got Its Flash Back," the Canonet was their first mass-produced camera.
Over the next 20 years they'd make a variety of models with different combinations of features, but they broke down roughly into three types. The first was the original Canonet, which had a bottom-mount film-advance lever and a selenium-cell meter surrounding the lens.
Around 1964 they redesigned the camera, putting the film lever on top and changing that huge selenium cell to a little bead-sized CdS cell (located at the 12 o'clock position above the lens glass). CdS required a little mercury battery to run, but it was more sensitive in lower light and took up a lot less space. Mounting it just above the lens glass, but still inside the barrel, allowed it to meter through a filter if you had one screwed on, so no screwing around with filter-factors. It also meant you didn't accidentally cover it with your fingers, as you might on a camera where the cell was located on the body.
The first with this new configuation was Canonet S in 1964, but a full line came in 1965 as the QL version. QL stood for "quick load" and it was a feature that made it easier to load 35mm film onto the takeup sprocket. Canon put the QL gadget on all their cameras in the late 1960s and early 70s. It works fine—I never understood why they quit making it.
They made three cameras of this, each with a different max aperture. The 17 had an ƒ/1.7 lens, the 19 had an ƒ/1.9, and the 25 had an ƒ/2.5 lens. These days the 17 is the most sought-after, but the 19 is only 1/3rd stop slower and everything else is the same. The 25, however, had a lesser shutter (a Copal SLV): it only went from 1/500th to 1/15th and there was no self-timer. The 17 and 19 had the Copal SV which went down to one full second.
There are also two Canonet 28s, which are different cameras entirely. I'll give them their own page if and when I acquire them.
In 1969 there was another redesign, called the New Canonet.