The zinc markers, I think, are my favorite finds in any cemetery. They were originally marketed as 'white bronze', and sometimes are mistakenly referred to as cast-iron. They weather well, and nothing grows on them because of the chemical reaction of zinc and organic material. It seems that families would purchase a marker and erect it with the first death. As more family members joined the reserve, the decorative panels on the other three sides would be removed and replaced with names and dates and epitaphs. Occasionally there will be granite footstones nearby.
Legend has it that many of these hollow monuments were used as drop points for bootleggers during Prohibition. Perhaps some were, once or twice, for small amounts, but the absolute bother of moving bottles in and out of the small openings makes the idea rather unlikely. If they were used at all it was more sensible that they'd be used as money drops.