History - Morse Codes
Interesting to see here that 77 which now means "Long Live CW" and/or "CW Forever" and/or "Long live CW and wishing you many happy CW QSO", way back in 1912 for a while meant "Message for you" — and 99 which later came to mean "Get lost" might have originated from the "Keep out" meaning toward "keep away" :-)
I suspect some possible error in the table of cut numbers, the 7 was likely mean to be -… but the T for 5 seems plausible given that 0 abbreviation was a longer dah, and that was certainly the case even until much later.