Canon F-1 Instructions Manual page 23

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Frame Counter
Each winding will advance the number of the frame
counter, indicating the number of pictures taken. When
the back cover is opened, the counter automatically
returns to the starting position "S".
Safety Device for Shutter
When the shutter lock lever around the shutter release
button is turned to the "L" position, the shutter button
is locked and will not move. This device is especially
useful in preventing an accidental shutter release of a
wound camera.
Attaching the Cable Release
The optional Canon Cable Release can be attached to
the F-1 by screwing it into the threaded hole in the
center of the shutter release button. Even if the shutter
lock lever is at the "L" position, the shutter will operate
when using the cable release.
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