Most one-bolt Campy SR seatposts have bolts that are somewhat smaller than 13 mm. I used to have a 1/2" wrench - but I don't remember whether it fit exactly (tightly) or barely not. However, since wrenches vary in how much "oversize" they are (to fit over an "exactly to size" bolt head), this may very with different wrenches.
Anyhow, while maybe nominally 13 mm, the Campy bolts are more likely 12.5 mm or so.
There seems to have been some confusion between 1-bolt seatposts (bolt underneath, easy to reach, but adjustment prone to slippage) and 2-bolt seatposts (bolts above, difficult to reach especially with modern, "low" saddles compared to a tall Brooks with lots of room under the frame, never slips)
Jan Heine, Seattle