Canon A-1 - Manual (Page 105)

Frame Counter
The frame counter of the A-1 is an additive
type. It advances to the next number,
indicating the number of frames already
exposed, each time the film is wound. It will
not advance higher than 38. The numbers 0,
20 and 36 are marked in orange, the latter
two as a warning of the end or near-end of
films having those numbers of frames. This
frame counter is also coupled with the
rewinding operation so that it counts back
the frame numbers as the film is rewound. It
automatically returns to “S” upon opening
the camera’s back cover.
The frame counter will conveniently stop
during multiple exposures so that several
exposures on the same frame will not be
counted as separately exposed frames.