What did George V refer to as a mixed grill?

King George V was a philatelist, one who admires and collects stamps, and the mixed grill is a type of U.S. stamp highly treasured by collectors. Stamps were grilled to discourage reuse or counterfeiting.

Sheets of stamps would be passed through a mechanical device that would make a small waffle-shaped impression in the middle of each stamp. This process was designed by Charles F. Steel and went into production in 1867. The grill would actually break the stamp paper’s fibers just enough so that, when canceled, the grill would create a blotter-like effect and absorb the ink, making it nearly impossible to soak out later.