Janet sent the card to Lesley for her 16th birthday
Janet Brittain bought the card for a shilling in 1967 before sending it to childhood pal Lesley Charlton on her 16th birthday.
It's since gone back and forth between the pair after Mrs Charlton started the quirky tradition by sending it back to Janet the following year.
It's thought the card has travelled around 9,000 miles in its time – and now costs them £6 in recorded delivery each year to make sure it doesn't get lost.
Retired teacher Mrs Brittain, 65, said: “I was away in Scotland when I bought the card – which is why it has a Scottish man with a long beard sitting on a penny farthing on the front.
“Inside there are boxes from 1 to 100 so you can tick the age of the person you're sending it too.
“I sent it to Lesley on her 16th birthday but didn't think much of it. It was only the following year that I was in for a surprise when she sent it back to me.
Each time the letter was sent they would add a drawing to the card
“That was it then, it carried on year-on-year and become a little tradition. Birthdays wouldn't be the same without that card.“
It carried on year-on-year and become a little tradition
Every time the pair send the card to one another, they include a letter and a drawing to each other, celebrating each year that goes by.
Retired children's book illustrator Mrs Charlton, 65, who still teaches art to adults, said: “We just send the card backwards and forwards and we always write a lengthy letter.
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“We've known each other since we were seven and we were incredibly close at one time.
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“It's only time and distance that's pulled us apart but this card has been a real link.
“I haven't seen anything like it since, it is unusual.
“It's a different kind of friendship because we don't see each other much, but when we meet we talk non-stop because there is a lot to catch up with.“
The card is thought to have travelled 9,000 miles in its time
The friends grew up together in Bishop's Stortford, Hertfordshire, but Mrs Charlton, a mother of two with three grandchildren, has since moved to Ely, Cambridgeshire, with her husband Paul.
Janet, who has two children and four grandchildren, now lives in Scarborough, North Yorks, with her husband Brian, where she runs a community arts centre.
They recently met up again however for a special reunion to mark their 50th MacTavish birthday – named after the MacTavish's Eternal card.
Lesley started the tradition when she sent back the card Janet gave her
Mrs Brittain added: “It's amazing how well the card is looking for its age. There are a couple of pieces of sellotape here and there but generally it's in good condition.
“We have to pay £6 each time to send it by recorded delivery however, it would be terrible if something happened to it.”