The adventuring guild was called
Ansur's Finest. Hence I thought it has something to do with the Dragon?
it makes sense, Ansur was supposed to be Baldur's gate's protector, so the adventure guild may be named after him. But it never appeared in the game

Concerning the letters, you probably mean the
Soft Letter? I remember with the release version, a dead male Flaming Fist can be found in Waukeen's Rest with a similar letter, but I don't know if there's a connection or if the letter independently belongs to the dead Flaming Fist's wife (i.e. another person) in Baldur's Gate.
soft letter is one of those that scratche's former companion was carrying iirc. and the one at the waukeen's rest i think is still there, it's a crude letter and it is on a flaming fist corpse, i think since it found it's recipient it can't be returned to the post office. but maybe i am wrong, i never tried it either way)