What you're looking at is actually a rectangular-shaped puzzle piece.Our best guess as to how it got into her sleeper is that Abner loves to put everything behind her head - and I do mean everything. She picks up books and drops them behind her head. Holds each end of a string in her hands and puts the string behind her head. Balls, blocks, food, clothes, you name it - it goes behind her head.
In this case, when she put the puzzle piece behind her head and dropped it, it must have slipped right into the neck of her sleeper and settled next to her diaper. Et voila - shittin' bricks.