Canon III-A - Manual (Page 34)

UNLOADING When the Film Winding Knob will no longer
turn, you hove come to the end of you film roll. NO NOT FORCE
IT. Turn the Reversing Lever from "A" to "R" (Rewind). Raise
the Rewinding Knob and rewind the whole film back into its
original magazine. When you have done this, you may open
the Baseplote and take out the film magazine. The film is now
ready for developing.
During the rewinding operation the Shutter Release Button turns
and stops the moment the film leader is pulled off from the
Take-up Spool. Rewind a few more turns so that the leader
will be drawn completely into the magazine.
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