Those sandboys

It's been a while since I've searched for "jolly as a sandboy" or "three jolly sandboys." Then I read a book that used "happy as a sandboy" three different times, so I've just searched again. Since the first time I read The Old Curiosity Shop (see chapters 18, 19), I've always wondered about the sandboys. All anybody ever says is to consider the saying to be equivalent to "happy as a clam." Today I found the fullest explanation yet (scroll down). I prefer my sandboys to be jolly rather than happy.