If it's original, it's leather.
But- soak it in Gasoline for 30+ years- and it looks like something else! I soak my new ones in Oil for a day before I install them, and I soak them in oil for a day when I take the carb off and go through it every year at the beginning of Spring. My car sits during the winter, so I go through the carb and check things every spring.
Your's looks like it has a chunk out of it, so I wouldn't try to re-use it. If it didn't I would have soaked that one, then flared the lip and put it back in!
As a hint:
I shoot all the gaskets with non-stick cooking spray before I install them. This makes it easy to disassemble the carb later as I don't destroy any gaskets in the process. This is a Holley trick BTW...

Enjoy!
CC.