If you have to say 1958, Thanks to the Original SLR the Kine Exakta in 1936, lead to the modern SLR lay out with the 1957 Asahi Pentax. And thanks to all that you get a Nikon F in 1959.

*If you ask that, the old Kodak brownie is a Box camera instead of a folder. So you'll have to think sometime before that even.