Bistro mugs in glazed ceramic

We all had coffee one day in those pretty iconic cups from bistros and café terraces. And for good reason, these cups were chosen for their great durability against shocks and their practicality: stable on the tray when serving and easy to store as they stack very well. Their base is marked with 4 grooves; I imagine it's to drain water that might stagnate during washing, but if someone can confirm this or explain its purpose if it's something else, I would be delighted. They are made of chocolate-colored glazed ceramic on the outside (a bit darker in reality than in the photos) and white on the inside. They are in very good condition, clean, no scratches, no cracks, just a small chip under the base of one and a tiny dot, a manufacturing defect in another. They measure 7.5 cm in diameter, 10.5 cm in width including the handle, and 6.5 cm in height.