Rule P (now solved!)
Moi un Mouton reported that the rule is based on day of the month.
Eraserhead identified the rule name: Presence in the forums, day last active.
Rule 1
wrc fan identified the determining factor: 3rd letter of user name, case sensitive
OrangeSVTguy identified the mapping and showed that there are flags for many dozens of characters. Since the rules for allowable characters in user names have changed over the years, we allowed for punctuation symbols that were previously OK but no longer legal, to handle legacy user names. So there's one flag for each ASCII symbol, and an extra flag for accented or other non-ASCII characters. (By the way,
OrangeSVTguy said that 'a' and 'J' have the same flag. Which user names did you compare?)
Nobody has posted the name of the rule, even though I gave you the first word. I hope our forums aren't limited to monoglots.