18 of 26 is where I am at present. I've added D, I, T and Z.
D is for Dubai, I for Istanbul, T for Tokyo and Z for Zanzibar.
Still puzzling over options for Q and W.
A friend has promised to bring back a couple of inflight magazines from his business trip so I'll be able to add some more letters, more purple mounted ones going forward - places I'd like to travel to.