One of the USA’s rarest single stamps is the 1868 1¢ Benjamin Franklin Z-Grill. Printed and grilled in 1868, the Z-Grill design was not used for very long, a couple of weeks at the most. That short production time is what accounts for it being so rare today. As explained in a previous article, a grill was a waffle-like design embossed on a stamp as a security measure to prevent reuse. See article: FAQ-What is a stamp grill?