When: 1949 Why: this is the first SLR (Single-Lens Reflex) camera to use pentaprism. Also it is the first camera to use 42mm screw lens mount (M42)
Pentaprism allowed non-reversed direct viewing from behind the camera, opposing to older SLRs top-view and reversed image. Pentaprism (or cheaper version: pentamirror) became an industry standard for modern SLR cameras. M42 mount, which was introduced for this camera, became an industry standard for 35mm cameras too. Later, M42 was called as "Pentax thread mount", "Praktica mount" and "Universal Screw mount" and was used on lots of SLR cameras.
Very detailed description of this camera can be found at the Rick Oleson's web-site.