It may not be unique to Hong Kong, but is definitely distinctive, that the (possibly forced) sale of one single flat is marketed through mass mailing.  This 5-7/8″ x 8-1/4″ card was mailed to Hong Kong residents in 2017 using Hongkong Post’s Chinese-language pre-printed Circular Service postage.  As with other, similar cards, this is fully printed on both sides, nearly entirely in Chinese except for fine print–which itself is extraordinarily fine.  Grade: 1