This system was actually the older order of the 47 kana characters (Hiragana & Katakana, here) and its first usage dates back to the Edo period. Around 1720, the firefighter groups Tokyo neighbourhoods around Sumida river had named themselves after this system. There was for example the "I Gumi" (1st group), "RO Gumi" (2nd group) etc.
The "I-RO-HA" numbering system was recited as a song to make it easier to memorise and goes like this:
I-1, RO-2, HA-3, NI-4, HO-5, HE-6, TO-7//CHI-8, RI-9, NU-10, RU-11, WO-12
WA-13, KA-14, YO-15, TA-16, RE-17, SO-18//TSU-19, NE-20, NA-21, RA-22, MU-23
U-24, YI-25, NO-26, O-27, KU-28, YA-29, MA-30//KE-31, FU-32, KO-33, E-34, TE-35
A-36, SA-37, KI-38, YU-39, ME-40, MI-41, SI-42//YE-43, HI-44, MO-45, SE-46, SU-47
Occasionally IJAAF units wrote on the tail the first kana of the pilot's name. In order to understand which of the two a kana on a tail is meant to signify, first the "I-RO-HA" system should be consulted. If the corresponding number is too high (over 20 for example) then it would mean that it probably comes from the pilot's name since units with a lot of aircraft didn't really bother with songs and prefered the Arabic numerals.
Inside the cabin of a "Hickory".