I forget which Unknown appears in which chamber, but either the ! Unknown or the ? Unknown appear in the chamber where only the A Unknown appears. You won't find any other forms of Unknown in this chamber, just the A, until the ! or the ? appears. For the other of the two rare Unknown, it appears in the chamber where only the Z Unknown appear. So if you find a chamber where you only find A, or you only find Z, those are the chambers, just be patient. I've found that each of the chambers has one rare form of Unknown, however I found them all after about 2 and a half hours.