This official Mayfair promo expansion was included as a retail give-away on a tile sheet titled: Limited Edition Promo Expansion Set #5.
Expansion consist of five city growth markers on five colors. They can be used instead of normal grey city growth markers. You place one of these markers on to the city, when you are using City Growth action. Now this city has one time demand for that color cube. When one cube of markers color, has been shipped to the city, then the marker is turned to the other side, which is exactly the same as a normal city growth marker.
- Steam: City Growth Special Contracts