This is probably the card that most frequently comes to mind when one mentions 'Official Postal Cards'. These cards were used by Postmasters for monthly account reporting to the Postal Savings Division headquarters in Washington DC. While not necessarily any scarcer than many other early examples of official postal cards, these have certainly enjoyed catalog listing for some time and are scarce enough to merit high demand and corresponding value.
The card is marked as form 5527-2, and was printed at the GPO in early 1913. The number printed is estimated at under 50,000. This is probably based on the period of use times the number of postmasters required to utilize the cards. These are facinating cards, but you have to have a large budget to study their usage.
Place mouse on image to see the reverse side of the card.