Because the keytop for the O key also showed a left-arrow symbol (from ASCII-1963, which had this character instead of underscore), a noncompliant use of code 15 (control-O, shift in) interpreted as "delete previous character" was also adopted by many early timesharing systems but eventually became neglected.
bɪkˈʌz ðə kˈiːtɑːp fɚðɪ ˈoʊ kˈiː ˈɔːlsoʊ ʃˈoʊd ɐ lˈɛftˈæɹoʊ sˈɪmbəl fɹʌm ˈæski nˈaɪntiːnhˈʌndɹɪd sˈɪksti θɹˈiː, wˌɪtʃ hæd ðɪs kˈæɹɪktɚ ɪnstˈɛd ʌv ˌʌndɚskˈoːɹ, ɐ nˌɑːŋkəmplˈaɪənt jˈuːs ʌv kˈoʊd fˈɪftiːn kəntɹˈoʊlˈoʊ, ʃˈɪft ɪn ɪntˈɜːpɹɪɾᵻd æz "dᵻlˈiːt pɹˈiːviəs kˈæɹɪktɚ" wʌz ˈɔːlsoʊ ɐdˈɑːptᵻd baɪ mˈɛni ˈɜːli tˈaɪmʃɛɹɪŋ sˈɪstəmz bˌʌt ᵻvˈɛntʃuːəli bɪkˌeɪm nᵻɡlˈɛktᵻd.