Matt Webb seems to have discovered an anonymous commentary on a text which may be tentatively identified as a sixth, and hitherto unknown and unpublished these twenty five years, Cities and Eyes story from Calvino's Le citta' invisibili: The Mirrored Spheres of Patagonia.
The basis for Patagonian civilisation, the discovery that turned a relatively simple agricultural community towards greater and greater complexity, was the perfection of their science of optics. Every citizen carried a telescope, and at intervals in their cities vast mirrored spheres were winched into the air. Smaller spheres were placed outside windows, and similar ones inside all rooms and scattered in all public places. Strung between cities and villages were magnifying lenses, repeaters, also winched up.
From what we're told it seems that this infrastructure allowed any citizen - from anywhere - to view any other point in the empire.