Tested and fully working.
New light seals
The Pentax ES was announced by Asahi in 1971, as the world's first SLR with 'fully-automatic electronic shutter'. It offers what has since become familiar as aperture priority automatic exposure (shutter priority AE was already possible with other cameras, such as the Canon F-1, at this time). Electronic shutter timing makes aperture-priority AE possible because it allows the shutter speed to be essentially continuously variable, not restricted to whole or half-stop values. Manually-set shutter speeds on the ES are mechanically timed, however (and so not dependent on the camera battery, so the camera is fully operable (though without metering) at all normal hand-held speeds, in the event of battery failure). The ES also makes an early use of a memory circuit in exposure control, to store the exposure setting in the brief period of 'blackout' when the mirror is moved to allow exposure.
Asahi  emphasised the advantage that, since the mechanisms involved in this mode of AE were in the camera body, not the lens, the camera could be used, in AE, with almost all existing Takumar lenses (in contrast, shutter-priority systems like Canon's had required the development of new lens mounts incorporating rapid stop-down mechanisms).