Callsign Restriction

Why can’t we use the letters I and O in our general aviation callsigns?

“ Numbers by themselves, and the letters “I” and “O” must not be used because they could be mistaken for a runway number . (2) The letter “X” must not be used because a sign with an “X” could be misconstrued as indicating a closed taxiway or runway. Source: FAA Engineering Brief No.”

Source: airport design - Why are Oscar, India, and X-Ray (O, I, and X) not used as taxiway identifiers? - Aviation Stack Exchange.

Same presumably applies to callsigns - is the potential confusion with the numbers 0 and 1 when being displayed.

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Benny has the right answer and a very close source. Here’s the N-number source:

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