Not so much “odd” as “richly designed,” and should be compared with 20307505 and 20307506.  OCT refers to Overseas Chinese Town, an eclectic section of Shenzhen.  This is a set of six unused cards, issued by China Post with pre-printed postage, and the scan shows the front and back of the set’s cover.  What appears as a rock on the front cover is actually part of one card, seen through a cutout window.  The back cover shows all six cards.  The reverse of each card has a coloured miniature version of the front’s design, taking up most of the message area.   Two sets are available.  Grades: 1