Bearing in mind that what we refer to as a broken tooth, is the result of a partial blockage of one of the tiny indentations around the edge of the die that form the teeth. its not unlikely that the hole started to block during 1915 then gradually built up, but during this period a small piece of the blockage may have broken away again, only to them build up again giving the variation in the size of the tooth, this may explain the out of sequence 1915 coin of Zoos