Thank you, Zaela, that makes perfect sense. So, I brute-forced from 1 upward and found that our guild ID is 315, but now it gets interesting!
Nobody from <Terminus Est> is on right now, but
both <Terminus> and <Terminus Rising> are showing up under ID 315 which looks to me as if "guild315" is simply translated by the server into a guild name ("Terminus"), and then the server executes a substring search, just as if I had entered "/who all Terminus" from the start.
The corroborating fact for the above is the fact that guild ID 612 seems to translate server-side to "Rip" as three different people (none in a guild named <Rip>) show up in the listing ("Riptite" in <Fires of Heaven>, "Riptorage" (no guild), and "Ripecloaca" in <Azure Guard>). Perhaps members of <Rip> have long ago gotten fed up with the problem and given up their guild?
Anyway, no joy, except now I have a really sore wrist from hitting shift-up and backspace over 600 times. Somewhere beyond that, a number will translate to "Terminus Est" and "Terminus Rising", but <Terminus> is struck holding the brown & sticky end of that stick.
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In any case, I'll have a look at clientlist.cpp, and see if I can make enough sense of it to devise some solution. I agree that the guild# trick seems to be custom, so it ought to be possible to improve it to where it works as I should hope it was intended.