It must be some func_door_rotatings, or similar entities such as func_platrots (which are rotating, rising platforms).

A func_door_rotating is a rotating entity that rotates back and forth between two positions.  You can set the degree and direction of rotation.  A func_platrot could be used for the same effect, but there is no need here.

Note that it would be much more difficult to create a rotating billboard that displayed more than two different adverts.  This is because a func_door_rotating can only have two positions.  You could create the illusion of more adverts by using a sequence of env_renders to swap several billboards, which appear to be only one billboard.  If you would like to know how this is done, let me know 

A func_rotating cannot be stopped in a particular position (it can be stopped by triggering, but where it ends up will depend on when it is triggered.  This is imprecise for positioning).

From my considerable and frustrating experience with rotating func_trains and tracktrains, I suggest that neither of these entities would work.

The source file to that particular map is not included in the SDK.

I could have sworn it was! Argh.