It's funny how you take things for granted that were so incredibly special just a few decades ago. Air mail for example; nowadays you wouldn't probably even know which stamps were for air mail unless it specifically says so on the stamp. I don't think there's many proper air mail issues anymore now. But you don't have to go that far back in time to find that things were very different. In the late 1910s, early 1920s the first air mail stamps were issued, and these were not general air mail stamps like so many of the later issues, these were issues for a few or sometimes just even one specific flight route! Which is really amazing when you think of it! Take for example this stamp here:
It is from a set of three air mail stamps issued in Czechoslovakia on 11 August 1920. Each value was issued for a specific route. This 28kc on 1000h was issued for flights from Prague to London. It came with a special air mail label with the route on. I would love to have a cover with those stamps plus labels on! If I get one, I'll show it to you! What I do have is a Swiss cover from the same period. It has the very first Swiss air mail stamp on.
This stamp was issued on 30 April 1919, and was issued for flights between Zürich, Bern and Lausanne. The covers from those period had a special air mail cancel on the back, because the stamps themselves were cancelled with an ordinary circular date cancel. I haven't scanned that cover yet, but I will show it to you soon, probably tomorrow.