I'm not sure that there would have been a real standard in those days, but from the British perspective:
By 1921, the RAF "Telephony Spelling Alphabet" had been adopted by all three armed services, and was then made mandatory for UK civil aviation, as announced in
Notice to Airmen Number 107.
https://en.wikipedia.org/wiki/Allied...lling_alphabet
The question is whether there was a similar convention on the other side of the Atlantic. This was the first I could think of.