The Benjamin Franlink Z-Grill Stamp has the unique feature that there are tiny squares embossed into the paper in the back of the stamp to absorb the ink. This is to prevent people from cheating the postal system by washing out the cancellation marks.
The Z Grill Stamp was bought for $935,000 in 1998 but later traded for a block of 4 Inverted Jenny Stamps which were bought for $2.97 million.
Only two known copies of the Z Grill Stamps are known to exist. One is owned by the New York Public Library while the other copy is the in the hands of financier Bill Gross (who traded for it with the Inverted Jenny block).