On Rod's map, off Union Island, is the smaller Palm Island.
This has some stamps under that name in the SG Appendix as does the small group of Tobago Cays.
Palm Island used to be called Prune Island and that name appeared as as a marginal overprint on the Princess Anne wedding stamps of the Grenadines of St Vincent (as did the other names on Rod's list).
Here are my two Prune Island "stamps":


I have similar pairs for Bequia, Canouan, Mayreau, Mustique Island and Union Island.
I guess these are private overprints but I don't remember ever seeing anything about them.