The S1A (or H1A in the US) was produced from 1964 until 1968. Later models had improvements to take new 50mm f1.4 lens and the back catch was attached to body instead of back. These models can be identified by an orange "R" on rewind knob (mine is one of the later models).

The S1A has a screw mount, but does not have any internal metering system, unlike the Spotmatic models. A completely manual camera without the need for batteries. A nice heavy camera that feels solid in the hand. Still a wonderful camera to use as it nears its 40th anniversary.

For information on early Pentax cameras take a look at this site or this site