I woudn't exactly call those "characters." I mean, yes, when you do a spectrogram of the signal you can see the difference between the high and low tones but that's kind of what a spectrogram is. The difference was clearly audible and the decryption of it was done by ear.
Not sure if you have seen the image in my (now approved) post #12044 on page #803
The UP uses octal chars 0-7 and also the division sign. I've decoded it and the octal characters match up with the binary numbers.