Illusration: Achilles sits in his tent while heralds of Agamemnon lead Briseis away. The Brygan Circle (imitators of the painter Brygos, lae archaic, early classical period; early to middle of the 5th century BCE)

I think that the bulge under the chlamys of messenger behind Briseis is supposed to be his right arm. I sent an inquiry to Professor Tim McNiven, an expert in Greek Vase Painting Iconography, who writes:

"It is his right hand, under his cloak, which he has thrown over his left shoulder covering both arms. I'd say it was for warmth, except the guy next to him is bare to the waist. The artist is trying something experimental, drawing the cloak this way, but he's not the best draughtsman, and so it is a little lumpy, and in an unfortunate place."

The other side of this cup is below, and shows the return of Briseis to Achilles. The man following her is an analogous position to the following herald above.