One item I left off the list of special progids is Directory\Background.

Recall that Directory is the progid for file system folders (a subset of Folder which represents all shell folders, both file system and virtual). Closely related is Directory\Background, which isn’t really a progid, but it is a place where shell extensions can register themselves. Specifically, it’s where context menu handlers can register if they want to appear when the user right-clicks on a blank space of a file system folder.