Although in Windows F5 is the common (though not at all mnemonically appropriate!) keystroke to refresh the contents of a window, I've seen at least two different applications that use F9. One is the much-maligned Lotus Notes (which is actually quite good if you can overlook the abysmal user interface ;), and the other is muCommander.

Since Lotus Notes has other non-standard conventions that apparently are borrowed from other places (such as Esc to close a Window) and because it's unlikely to be a source of influence for many applications, I'm betting both apps borrow from a common source (even if indirectly). What is that source? Where does F9 as the refresh key come from?