s40s are the side zip and the s21s are the back zip. besides that, they're pretty similar.
breezy: yes, the top left are the ones i was selling. they are no longer for sale :B. the middle one is indeed the destroyed horse. i am not sure why the leather is so soft on the s21w compared to the s45m.
From top to bottom, left to right:
Carpe Diem S21M Dark Brown Double Soled Horse
Carpe Diem S21W Black Destroyed Horse
Carpe Diem S23M Black Double Soled Calf (?) Actually not...