Original: An alternative, the slashed zero (looking similar to the letter O except for the slash), was primarily used in hand-written coding sheets before transcription to punched cards or tape, and is also used in old-style ASCII graphic sets descended from the default typewheel on the ASR-33 Teletype.
Simplified: The slashed zero, looking like the letter O with a diagonal line drawn inside it, is used in old-style ASCII graphic sets descended from the default typewheel on the venerable ASR-33 Teletype.