The former Postmaster general of the United States is a popular figure to feature on many US postage stamps. The most valuable of all Benjamin Franklin stamps is the 1c Benjamin Franklin Z Grill, sometimes just referred to as the Z Grill. A grill is an embossed pattern on the face of a stamp used to prevent the reuse of stamps. The US Postal Service used several different types of grills in the early days of the office as they experimented with the best methods to stop fraud. The Z grill was not widely used, but there are George Washington and Abraham Lincoln stamps that also feature the pattern. The Benjamin Franklin Z Grill is the rarest, with only two known surviving examples. This stamp was last sold at auction for $935,000 in 1998, though one was more recently traded for a block of four Inverted Jennys valued at 3 million dollars. The US Post Office released several million sheets of Inverted Jenny stamps in 2014 for the 95th anniversary of the misprint. Among the intentionally inverted stamps were 100 non-inverted sheets of stamps, a teasing tribute to the original mistake. The sheets of ‘Upright Jennys’ are collectors items in their own rights, selling for as much as $50,000. However, this publicity stunt did cause some controversy, as the US Post Office is not allowed to intentionally print errors.