Ah, finally something to apply my vast and totally useless mass of knowledge to!
White dog dung is indeed from doggies munching bones-but it takes time to turn white. Once it has done this, it becomes 'the pure', and was used in the production of leather for many years.
Wretches(well, what else are you going to call them) would roam the streets with tin buckets, snagging these enchanted turds which they would sell to tanneries. They were called "collectors of the pure" and are noted as same in municipal documents.
My Grandfer, upon whom be peace, told me of this as he mused on how life had been when he was young, and I've seen it elsewhere as well.
When I was a child, our spaniel(and may God keep her, too, she was a saintly doggie) used our yard as it should have been used by a dog, and I saw dark dog rockets turn white as a few weeks went by. This is what prompted my Grandfer to explain this to me-he liked to talk.