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iron

In Great Britain and the United States, ?–20ᵗʰ 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.)