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 .