Thats an ancient factory MkV trigger from before they put the safety on the bolt shroud. You may have enough slop in the linkage on the left side that it isn't fully dropping the bolt stop. Try to pull the trigger and then get ahold of the bolt stop pin with needle nose or something and try to gently pull it the rest of the way down to clear the bolt. Be gentle because if something breaks on that pot metal trigger Weatherby will charge an arm and a leg to retrofit it with the new style shroud and trigger, I'm speaking from experience in that area.
