Hi Mike To answer your question I personally have only seen four of the Aa type . Below is a picture of the four types I have come across . Top is Ac with the 1 directly over the tooth [ I have seen about six of this type]. Next down is the normal common type Ab with the 1 slightly to the left of the tooth but the 1 its self is almost touching the tooth . Third down is very similar to Ab but the 1 is more to the left of tooth and importantly has a gap between it and the tooth { not listed by Gouby as its to similar to the common Ab type} bottom is Aa with the 1 directly over the gap between the teeth , all three of the rare types have that space between the 1 and the row of outer teeth, as Pete has already said