It's always great to be able to get a key item from a set. The one that makes the set interesting. I was therefore very happy to lay my hands on this stamp here.
It belongs to a set of provisionally overprinted stamps for Crete. These stamps were issued because the properly overprinted stamps had not arrived yet from London. What's so special about it? Well, have a look at this:
Now, before all these overprinted stamps were flooding the market, the 20L red was replaced by a 20L orange. So when the then current stamps were overprinted, naturally the 20L orange was used. However, a few sheets of old 20L red stamps were also overprinted. These almost immediately fell into the hands of speculators and were soon invalidated. They were never sold over the counter and are now relatively much sought after!