As the complete sceptic I am, I have my (severe) doubts, but my grandmother used to swear that the following story actually happened to her sister when she was a young woman. The great aunt existed, alright - so apparently did the letters (though I've never seen them). The rest is highly doubtful, ...
My grand aunt Emily was a highly intelligent woman. She also considered herself somewhat of a revolutionary in her youth and used to hang out with a crowd of local communists (of whom there were quite a few more back in the day). Among them was a talented young playwright, James, with whom, aged 19, she fell deeply in love - and he with her. The love letters she received from him were the most beautiful my grandmother had ever seen; despite never having met their author, my grandmother always spoke of him as though she was a little in love with him herself.
When he wasn't writing gorgeous letters, however, my grand aunt Emily's young lover was also a bit of an adventurer. The small town into which he was born was not wide enough for him, and he yearned to travel and see the world. Before James left he promised my grand aunt that he'd marry her if both of them were still single and childless at thirty and that he'd come to get her at their local on the 3rd of September 1943.
Predictably, the attractive Emily did not stay single for long. She had a couple of affairs with artists and wannabe-revolutionaries but eventually got sick of having her heart broken each an every time and married a somewhat boring but dependable local man.
While Emily liked her husband, though, she didn't love or desire him much - nor was she particularly ravished about her life as a 1930s housewife in a small town. She found her escape from this life in the letters her young playwright used to send her, and used to fantasise about life by his side rather than that of a local accountant. When she felt particularly bored she used to imagine that she'd meet the love of her youth on the agreed day and that they would elope together.
One day in the spring of 1943, Emily was again reading through one of her favourite letters when she spotted something she had never seen before: At the bottom of the last page, something had been written in pencil: "I will be long gone by the 3rd of September. Farewell, my beloved, always remember!"
James died as a soldier in WWII in May 1943. The letter still exists but there is no trace of such an addendum.