In 1893 the postal authorities of New Zealand tried a new approach to stamp printing. On the gum side of regular stamp issues they printed a series of advertisements for a wide variety of products: soap, foods, medicines, and so on. The ads were printed on panes of 60 stamps in three settings, using values ranging from the 1d to 1/s.
Collectors can find examples of individual stamps with adverts on reverse with a fair amount of ease. Here is a very nice block of four mint stamps that we currently have in stock.
The settings can be distinguished from one another based on various criteria. The first setting shows ads inverted relative to the face of the stamp (for sideways ads the bottom of the ad is on the left side of the face of the stamp). In the second setting the ads are upright. The third setting is similar to the second, but replaces all the Cadbury ads with ads for Poneke products.
Ads were printed in different colors as well, and this can be helpful in identifying the setting for individual stamps. See the Stanley Gibbons Stamp Catalogue for sub-listings of individual stamps with advertisements. The Campbell Paterson Catalogue of New Zealand Stamps is helpful for a more detailed description of the settings and panes.