Neither rain nor snow nor gloom of night ... but maybe world war one?
My guess is somebody stole some mail, or neglected to complete their day’s work, between Bath where it was posted and London where it was supposed to go, and then a century later some descendant of the thief found a bag of letters in their shed or attic, and just threw them into a mailbox somewhere. Since they were valid from the time of posting, they just completed their journey.
Caption:
Huma: “Anthony. Some mail came for you.”