Hi Mark
No, I'm sorry, I don't know exactly when Castlewood cemetery opened, but if William Cranston's death certificate states that he was buried there on June 20th, it's probably telling the truth.
I don't know the financial circumstances of your ancestor, but it's worth remembering that not everyone had a headstone. If a man left a widow and family of young children, paying the rent and feeding the children took precedence over buying a headstone..
Regards,
Lesley