Beauties, Jim! Congrats Jared! You are getting a sweet looking brush. I have the Tuxedo knot in my 30 DC LE brush, and it's a winner!
At first I thought that it looked like Osage Orange, aka hedge apple. But it wasn't the right shade of yellow. Locust and Mulberry looks sort of similar to Osage, but a little different shade. Any of those woods will make a handle that should last a long, long time. It may eventually darken when exposed to sunlight, but that's the allure of some woods.
That's it, Black Locust posts will last longer than the holes you put them in! The end grain in the first pic was the giveaway. As far as domestic woods are concerned, it's one of the hardest and heaviest, and dimensionally stable woods in North America. Some smaller trees that don't grow large enough to use for lumber, such as Osage Orange which will beat in on all counts, but Black Locust shave brush handles could last for centuries! Beautiful!!!