Jessa Krick here from the Rosen House at Caramoor. Many of our paintings here are labeled with label holders like these:
We use printed paper labels on cardstock. These are easy to design, as well as change as needed (whether discovering errors or updating information). The label holders are not heavy, so they are easy to mount and move if needed. Although they look modern, it is not incongruous in the historic site setting. Just be conscientious of font size for ease of reading, especially if the room is dim.