I checked this book out of the library numerous times between 1964 and 1966. I don’t think it was new at that time. A little girl and her family relocate to a home that used to belong to her grandmother/great-grandmother. While remodeling the house, a door that had been covered over was discovered. The hidden room had apparently been used as a hiding place during Indian raids. There is also a large, hollow tree in this story. I cannot recall if a leather trunk with the grandmother’s initials in nail heads, was found in the hidden room or the tree, but inside is a beautiful doll with beautiful clothes. One outfit was definitely an elegant riding habit. It is possible that the little girl was named after the grandmother so had the same initials as those on the trunk.
I hope this rings a bell with someone. I have tried, unsuccessfully, in the past to discover this long lost, well-loved treasure from my youth.