I have reason to believe that is an embossed reptile skin. I used to have a desk blotter in a very similar looking skin, and it was simulated exotic. Also, the fact that the exotic skin doesn't cover the entire case--witness the calfskin bottom--leads me to believe that it is merely faux.
That very well might be. But that would also mean that it's not likely from the 1920s or 1930s. I don't think there was very much or any, alligator embossed leather back then. Pretty much always the real thing if it was an alligator look.
Do you know how old your desk blotter is? If it's from the 1920s it would rare if embossed.