The Jewelled Snuff Box — Alice Chetwynd Ley

England, 1802: The lives of five people depended on the hidden mystery of The Jewelled Snuff Box

Jane Spencer fell in love with a man who had no name. She found him lying half-dead in the snow, the victim of highwaymen. But when he awoke he knew not his name nor the sight of his only possession: a magnificent jewel-encrusted box.

And then, when Jane came to care for him, he disappeared as suddenly as he had come into her life -- leaving only the box behind. Jane had no way of knowing the box was the all-important key to a mystery -- and the solution involved her best friend, her titled employers, her vanished lover, and her own destiny...
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Corgi UK Edition:

When an old bachelor marries a young wife he has reason to be a little concerned. The Earl of Bordesley was no exception. Celia was devastatingly beautiful, and before marriage had been one of the Toasts of the Town, but she wasn't the easiest young woman to live with, and when my Lord Bordesley employed Jane Spencer as a companion, he hoped his wife would benefit from the friendship of someone her own age.

But when Celia and Jane met, their astonishment was mutual. They had known each other from schooldays, when Celia had been as spiteful as she was now, and Jane had realised her life with Celia was going to be unpleasant. It was quickly made worse when she re-met, in Celia's company, the man whose life she had saved on a snow bound highway in Kent - the man who had given her a jewelled snuff box...