The postcard is date-stamped 1929 and was posted from Essex.
A couple have become the unwitting recipients of a postcard delivered 79 years after it was first posted.
Arthur Davies and his partner June Nicolopolous received the item at their home address at Lacon Road, East Dulwich, on Thursday lunchtime.
It was addressed to a Mr & Mrs Richardson, date-marked 1929 and posted from Burnham-on-Crouch, Essex.
The Royal Mail said it was difficult to "determine exactly" what happened to the postcard for all this time.
"My initial thought was that it was a hoax," said Mr Davies.
This was posted in 1929 so I would say these two girls would [probably be] in their 90s now, if they're still alive at all"
"But when I looked at the envelope it was in, I noticed it was Royal Mail and it said that this item had been delayed in the post.
"You expect sometimes letters to be a couple of days late but 79 years is taking the mickey a bit."
The postcard has a picture of the war memorial at Burnham-on-Crouch on the front.
"It looks like the picture was taken a couple of years after the memorial was first built because it looks absolutely pristine."
The writers of the postcard, possibly called May and Nel, simply say they have arrived safely and ask Mr & Mrs Richardson to write soon.
"This was posted in 1929 so I would say these two girls would have to be over 21 to go away on their own," said Mr Davies.
"I'd say they're in their 90s now, if they're still alive at all."
'Impossible to explain'
He said they would keep the letter in the hope that a relative may claim it back.
A Royal Mail spokesman said: "It is impossible to determine exactly what has happened to this item of mail.
"It is very unlikely, however, that it has been in the Royal Mail system for that length of time.
"The most likely explanation is that the item has been re-entered into our postal system and then delivered to the correct address."