Toronto. The SLR design dates back at least a century. Checkout the older Graflex cameras. Ken Metcalf on the west coast of the States published a newsletter devoted exclusively to Graflex models of all sizes. There is even a website!
Recently John Linsky sent out an email with a wikipedia link to the history of the modern digital variety of SLR – the DSLR which is the choice of camera makers like Nikon and Canon for models intended for both professionals and serious amateurs. I use a mirrorless Sony camera with an APS-C sensor since an adaptor lets me use some Leica gear I have on hand.
My first serious 35mm camera was the Exakta Varex IIa shown above. I bought it along with two other lads working at the site in Labrador. We bought from a dealer in Drummondville the summer of 1958. Leica had come out with the M series in 1954 but I had only seen IIIf Leicas with their tiny f/3.5 lenses and squinty viewfinders. Besides an Exakta with an f/2 lens and many more features was over $100 more expensive!
About 14 years later I switched to a Leica M4. My eyesight wasn’t up to doing any critical focussing with my Exakta by then, especially in dim light. The M4 had a bright viewfinder with a nearly life-size image and a crisp accurate viewfinder.
The SLRs of the 1950s and 1960s used preset lens apertures and manual mirror resets after exposure. The Japanese camera makers were first to come up with an instant return mirror SLR design and as they went from imitation to innovation, they decimated the other cameras on the market including German cameras.