"It was rather hard in those days because there was no internet or anything like that. It would take about six or seven weeks for a letter to arrive.
"In the beginning we only really got about two or three letters a year."
This meant things could change dramatically between a letter being sent, and reply received: "especially when we got married, and had families, there was quite a lot happening in that time".
But communication never lapsed between the two, who have now shared most of life's milestones with each other.
"She's more like a sister really than a pen friend, we just got on so well together. Seventy years is a long time and such a lot has happened in that time.
"We both lost our husbands, I've lost a daughter, she lost a son. [With] things like that, her friendship means a lot to me."
Mrs Martin - who moved to Hastings in the 1950s - was unsure how many decades ago it was that the pair first met, but remembers "it was wonderful".
"We didn't realise until we met that we had so many other things in common. She liked doing the things that I liked doing, there was just such a lot that we found out."
The pair have since made many trips to visit each other.
Technology has made a big difference to how the pair communicate now, which is mostly through email.
Mrs Martin said on their November 28 anniversary she planned to give Mrs Finch a call - something which seemed impossible when they began writing.