Flat grave markers shall not be less than twelve inches by twenty-four inches in size, nor more than fourteen inches by thirty-six inches in size, and shall be installed upon individual grave space. When double markers for marking two grave spaces are desired, a minimum size of twelve inches by thirty-six inches and a maximum size of fourteen inches by seventy-two inches will be allowed. All flat markers must be of bronze alloy, aluminum alloy, granite or marble. Markers made of entire concrete composition will not be allowed. All flat markers shall have a concrete border as above described.