I have a nice Sierra Leone cover that belongs to my Grandpa but its in a bag in my house ( with a couple of albums ) ill scan it later, it always fascinated me when I was younger, not worth anything at all but still something I will always keep since I was so drawn to it when I was younger. In relation to this post it is self adhesive unusually shaped stamps on an airmail cover as far as I remember! Diamonds and everything on the cover West so I think you will be interested!
By coincidence you may get George I and George II, rarely on cover and hardly with diamonds but no KGV.
Once upon a time some poeple collected just triangular stamps and when they found some popularity all different shapes appeared.