I'd have to submit the Topcon Re Super/Super D slrs of the mid '60's. If I'm not mistaken, it was the first slr with wide open through the lens metering. The Topcons copied the Exakta 3 claw bayonet mount after WWII, had interchangeable finders (prism, waist level) and placed it's Cds meter on tiny slits on the rapid return mirror which allowed you to change finders without losing meter capability. It came out with one of the first 300mm 2.8 telephotos as well as incident metering capability via a white hemisphere attachment which screwed into the 58 mm lens. It was a classic in that it was one of the first system cameras (motor drive, microscope attachments, etc.) and was widely known. The US Navy used these cameras as their official photography system up until the late sixties. Being wedded to the comparatively narrow Exakta lens mount was probably it's downfall since it severly restricted lens design. A very rugged camera and I still have one in working order.