Canon AT 1 - Manual (Page 38)

Frame Counter
The frame counter is an additive type
which counts one frame every time the film
advance lever winds the film. When the
camera's back cover is opened, the frame
counter automatically resets itself to the "S"
position.
While rewinding film, the frame counter
counts back the frame numbers. The starting
position "S", 0, and the even numbers 2 to 38
are displayed by the counter. Numbers 20 and
36 are marked in orange to call your attention
to the end of commercially available film
cartridges. The frame counter cannot count
higher than 38.