Perhaps the painter who used this palette made paintings on board that they rested right on it, forcing the use of the palette itself to the borders, where the paint build up is about 3/4 of an inch deep, creating a a nice edge for a panel to push up against.  Or, perhaps they scraped and cleaned the rest away but left the border as history. In either case, pretty beautiful to me, and with a great sense of future possibility about it.

15 1/4" t x 12 1/8" w x 7/8" d. Hardened oil paint is very stable and palette is very sound, no cracks or splits or issues.