That is quite interesting. They must have classed the first matics as a Deluxe auto.
Funny thing is that at the same time the Morris 1100 was YMGS1 and the Morris 1100 Automatic was YMGS3. So at the same time that they kept the same prefix on the Mini they changed the prefix on the 1100.
That would explain why the Mini Kingdom book does not list a seperate number for the early matic.
Another thing I have noticed it nearly all the auto Minis and 1100s that I have seen are Snow White (on the plate)

In Watto's writeup on the `matics in TME, he mentioned that the very first ones were called Mini Deluxe automatic, in fact I think there was a pic of the boot.