"The symbols that most often occur side by side with the S&XB are the hour-glass, the bell and the coffin. The symbolism is pretty clear."
Yep, I think so. I still see it as part of the Dance of Death tradition (the Danse Macabre was often carved on church walls). Death was much more of a presence in the lives of medieval people than it (he?) is in our lives, and their art/symbols show a preoccupation with, indeed, almost a celebration of, death.
Like I say, though, well off topic. I could talk about the Danse Macabre all day, but I think I'd better be quiet now. :)