It's a swap of all 12 x 4 car 331s moving from East to West, and 9 x 3 car 331s moving West to East. So the West gains 21 carriages, but loses 60 carriages (15 x 4 car Class 319s), so a 39 net loss...although the 331 carriages are longer at ~23m compared to 20m 319s.
EDIT: also the single Class...