Had a bit of a look around home, nothing on codes but Chinese dictionaries also use a similar system with one or more indexes eg
1. Characters in order of radical (base of character water,fire,man etc) and number of added strokes being the base order and numbered
2. Total number of strokes to give number as per 1
3. Phonetic (a number of systems) to give number as per 1
I have a couple of Chinese/English dictionaries and they both use this system with different four digit numbers (due to characters being included/excluded).
The system was probably standardized and a major library should have a copy. Censorship requirements in WW1 would have ensured copies were widely distributed in the none chinese world.