To the best of my knowledge, this Egyptian postage stamp, along with the two other denominations in the same 1937 series (5 mills and 15 mills), are unique in a very specific way. My puzzle to you is: what makes these stamps so special?
Place your guess by commenting below (one guess per person). If you are the respondent with the correct answer, your ‘prize’ is that you may ask me any question relating to the themes of this blog and I will write a separate post on that subject. Happy hunting!
Edit: Well, that didn’t take long. In just over 20 minutes, Dan Milton successfully determined the answer. In case you still want to figure it out on your own, I won’t post the answer here in the main post, but you can find it in the comments if you’re stumped. I will follow up with some analysis later.