Sentence:  They [19th-century headstones] instead normally recorded only the names and birth and death dates of family members buried in a plot over a period of seventy-five years, a century, or even longer. Stones that William Bigham Sr. made in Pennsylvania, especially the <WSD>ledger</WSD> stones but even small headstones, often seem intended to provide space for a similar record. 
Meaning of the word marked with <WSD> tags:  A large, flat stone, especially one laid over a tomb.