I have all of the currently available black covers, they're all numbered in order of publishing up until "Thief of Time" and after that the backs are blank where the numbers should be. I just checked and my copy of "Thud!" definitely doesn't have any number printed on it.
As to why yours has a 1 on it I could make a guess at a printing error, the copies for the city watch collection were probably printed and bound especially for it rather than taken individually and sold together out of the current stock of black cover editions. I'd imagine that this is something unique to that collection.
It's all about the sun master, white snow and red blood and the sun. Always has been.