Historic British post boxes. There was a period where blue post boxes were briefly in vogue. These boxes, such as this George V example, which stands outside Windsor Castle, were for air mail. They were introduced to tourist venues between 1930 and 1938. However, having separate air mail boxes proved to be too costly and in 1938 the Royal Mail decided any box could be used to post air mail. The Windsor box is now a tourist attraction only, and can no longer be used.