 Originally, the SA-mount was a dual-bayonet mount with inner (SA-IB) and outer (SA-OB) bayonets, the latter being a feature intended to mount heavy telephoto lenses, but never utilized by Sigma and consequently dropped with the release of the "SD14" in 2007.There were two precursors to the introduction of SA-mount cameras, the manual-focus Sigma Mark-I in 1976, which still featured a M42 screw lens mount, and the "SA-1" of 1983 with Pentax K-mount.Mechanically, the (inner) SA-mount is similar to the Pentax K-mount as well, but with a flange focal distance of 44.00 mm, identical to that of the Canon EF-mount.