Tooling marks it looks like. They advertise them to be weld free, which means, it starts life as a flat disk of ss304, it's then punched and stretched over a die with giant hammer mill. Sorta like the way they make pots. You can polish it up though, with a little elbow grease. I highly recommend Flitz Metal Polish. Or you can leave it alone, if you like that raw industrial look.....another point to consider is highly polished surfaces show scratches more. FWIW I have a Stout Tanks Fermenter which I've bee happy with, that is polished from the factory like a mirror, and the few scratches I've put in it are very visible.