I hadn't even found the coffin - I though it was a bridge.
I was thinking about this last night, I'm quite convinced the things are "Sacred hearts" and it looks as if the arm is adding one to a heap of them. It's a little bit mediaeval looking - Bosch-like sort of thing.
I had wondered if it was a memorial for some folk who had died in a plague... perhaps a whole family, but that the last one being added is the most missed, the eldest son or the most recent baby?