The control number on this Phonecard (BTD004 / D6a) has been applied to an inverted card, i.e. when the card is turned over like a page of a book, the number appears inverted in the top left corner of the reverse, rather than the right way up in the bottom right corner as was usual at the time.
On first glance there's nothing rare about this green definitive card, but on a closer look (at the cards track), did this Phonecard just have 10units or 50units or more? The burn mark from the cardphone would suggest so. The cards control number is: 586K21945.