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,[2] but with a flange focal distance of 44.00 mm, identical to that of the Canon EF-mount.
All Sigma SLRs and DSLRs can use manual focus lenses too, which have no electrical communication between body and lens.
Despite the physical similarities Pentax K-mount (PK) lenses can't be fitted straight onto the SA-mount as well.
(A similar Sigma MC-11 EF-E adapter is available to mount Canon EF-mount lenses on Sony E-mount cameras.)