Sorry I can't cite any sources. None that I know of mention this and therefore there is no agreed-upon method. I've had many discussions with other transcribers about this issue. Some have suggested using a flat or a sharp before the fingering, but a flat in front of a finger number 2 would read as a downbow for the next note. Some people have used word-sign expressions before the note. I've personally used dots 12 after a fingering for the Low and a dot 1 after a fingering for Hi. Of course, whichever method you land on, add a transcriber's note to explain what you're doing.