Musketjon, a good thought, but I'm hoping that's not it. I lightly spray my casings then roll them in a towel before depriming/resizing. I then check and correct case length before tumbling. That's a lot of handling not to have noticed these dents. There's a chance I got carried away during one of my reloading sessions and oversprayed. I will certainly be on the lookout now.

Still waiting for the right day to run some more rounds through her...