iron

In Great Britain and the United States, ?–20^th century, a unit of thickness used in grading heavy leather, one iron = ¹⁄₄₈th of an inch.

Sources

3.2.114 Iron. A term used for measuring thickness of sole leather. One iron equals 1/48 inch (0.53 millimeters).

Federal Test Method Standard.
Leather, Methods of Sampling and Testing.
Federal Test Method Standard No. 311 (January 15, 1969).
Washington, D.C.: General Services Administration.
Section 3, definition 3.2.114. (Document is loose-leaf.)