Can you spot why the lower left hand stamp is "different"? (hint : have a look at the top of the helmets!)

Answer: This is known as the "bird on helmet" variety and although not quite what we know a robin or starling to look like, it is close enough to be named the way it is!

Interestingly, although the value of this one stamp is enhanced when on its own as a single stamp, the presence of the other 5 in the Block make this a far more attractive proposition to a "variety" collector, as it allows the error to be compared and to stand out from the normal stamps!