In my original post about this story I asked if there had been other "young couples". No one answered, so I will. Yes. In the scene immediately preceding KM's entry through the French windows, when Marcella offers herself to Priest (thinking him the "master" of the estate) and the narrator threatens to throw him into the fire if he doesn't summon the master, Priest says: "It was not the lady, sir. It was you. I want that understood this time." He means that it was not Marcella's offer of her body but the threat of physical injury to himself that causes him to reveal the master "this time". Priest is afraid after KM enters, but not of the narrator (p-46). That the photo of the staff is "dog-eared" (p-40) indicates that it has been handled often. Priest also retrieved it from the trash after the narrator had discarded it. Priest has no need of it, but the next couple might. What became of the other "young couples"? How long has this been going on?
Roy