The final restingplace of the unborn
The box is believed to have been used as the final restingplace for an unborn child, a fetus. In miscarriages and stillbirths, people have been known to place the fetus in caskets or small homemade coffins and hid them in or around churches so that the deceased soul could be in a holy place. This is because stillborn children and aborted fetuses cannot be baptized and therefore have not been able to be buried in consecrated ground.
The casket has etched the letter A on one side. Maybe to tie the little one to the family, if the lost child did, after all, make it to heaven