Also the name Yakuza meaning Japanese gangsters comes from "893" (yattsu, ku, san). This name originates from a card game called oichokabu, which is usually played with a deck of hanafuda "flower cards". A player's score in this game is decided by adding the scores on several cards and using only the smallest digit. So because 8 + 9 + 3 = 20 = 0 points, "8-9-3" means "no points". So the original meaning of yakuza (ya for yattsu, eight, ku for "nine" and za for san, "three" was "no points" or "useless". Later, this meaning changed to "useless people" or "gambling people."
Thought I remember reading this.
http://www.sljfaq.org/afaq/yakuza.html
Please cite your sources next time.
So add the numbers "893" to the murder scene, subtly. While the Yakuza would never give themselves away like this, someone framing them might.
It's just slang, it's not like it's actually withheld and used. I don't even think posers would do something like this.
Yakuza is just slang for gang (just like katana is just a single-edged sword), and the media is told to call them 暴力団, boryokudan, or something pertaining to a group of violence, and the real gangs call themselves 任侠団体, ninkyo dantai, or something like a honorable/chivalrous organization.
And then, these terms are just generals; there's three main gangs, and other smaller ones (that are being taken over by the larger ones) nowadays. It would be best to refer to them directly by name.
As far as I know, there's no ritualistic killing for Japanese gangs. There's ritualistic self-harming to show apologizes and personal wrong doing, and killing/sacrifice related to the general Japanese culture. If you're basing this in Japan, there's very distinct things that can lead a killing to a gang, like if a gun was used (gun laws are very strict). In the states or elsewhere they operate like any other large gang with big resources--there's very little different, unlike what most media tries to paint.
I suggest you just work in connections and typical setups; connection to/from victim, area of killing, witness/evidence.