On the reserve side of the granite monument for the plaque of Deacon William WARD is this matching plaque which reads:
Erected 1923 by Artemus WARD of the Seventh Generation.
I returned to the Marborough Cemetery Inscriptions book and looked for any mention of this plaque and the one on the reverse side. The book is copyrighted 1908. I found no mention of it so we can be reasonable sure that this plaque's date is correct.
It pays to photograph everything on all sides of a grave. You never know what you are going to have to use for evidence.