Toronto. The SLR concept is at least a century old now. The Graflex comes to mind. It was an ideal design for interchangeable lens cameras since one could view through the lens rather than needing a separate viewfinder for each focal length. When I went to Labrador in 1957, I remembered the big ads for the Exakta camera. Although Leica marketed the M series cameras in 1954, I had seen only the IIIf cameras with their little 5cm f/3.5 normal lens and their squinty viewfinder with the rangefinder next to it.
When three of us in Labrador bought good quality 35mm cameras in 1958, we all chose Exaktas with their big bright waist level viewer and the eye level pentaprism viewer. The Exakta body and an f/2 normal lens cost over $400 even with a big discount while Leicas were $100 or so less. Exaktas were made by Ihagee (IHG) in Dresden, then in East Germany. The designer was left handed so the Exakta winder and shutter release were swapped right to left.
The Japanese brought out the first SLR with an auto-return mirror and took off in the marketplace (German SLRs at the time used preset stops and manual return mirrors). Professional cameras adopted the SLR design and the rangefinder fell out of favour. I discovered that, as I grew older, it was harder to focus the SLR, especially in dim light so I bucked the trend and changed to a Leica M4 with its bright, almost life size viewfinder and sharp focussing range finder.
In recent years the digital era has grown in leaps and bounds. Professional digital cameras – Digital SLRs, or DSLRs replaced the film versions and top lines like Canon, and Nikon took over with their APS-C and full size sensors. The cameras used the same lens mounts as their film brothers so a DSLR could use older lens. My friend John Linsky found this interesting Wikipedia page which outlines a history of the DSLR. Have a read.