A Ki-54Ko belonging to the "Rikugun Koku Tsushin Gakko" (Army Aviation Communication School) just before taking off on another training flight. The school was located in Mito, Ibaraki Prefecture. The katakana "ロ" (RO) above the unit marking indicates that this is probably the second aircraft of the school or the second aircraft of the type belonging to the school. It was very common for IJAAF schools to use the more traditional "I-RO-HA" numbering system for their aircraft.
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"
A Ki-54 belonging to the Mito Rikugun Hiko Gakko (Mito Army Aviation School) is just back from a shooting training flight for machine gunners.