Thanks, Lisa,
The inscription on the other side (south side) reads, "In memorium of Adam Hall died Oct. 4 1881 aged 77 years Also Catharine Jeffrey Jeffrey wife of the above died Feb. 26, 1882 aged 75 years." Catharine was another daughter of William Jeffrey and Mary Lyall. From my records, Mary Lyall is buried in the family plot of Adam Hall.
The record in the register states, "Mary Lyall, wife of William Jeffrey, age 83, born in Scotland, died in Peterborough, cause of death, old age, date of death 26 May 1857, by Rev. Mr. Reger." John M. Reger was the Presbyterian minister in Peterborough at the time.
As I mentioned in my initial post, William Jeffrey is reputed to be buried in the family plot of another son-in-law, George Gillespie in a cemetery in Keene. But no records have been found to confirm this. No tombstones or gravestones have been found for George Gillespie or his wife Isabella Jeffrey Gillespie, either. So that is the dilemma I am attempting to resolve.