Thanks for that
Think I may have answered my own question
In some parts of the world, top-grain thicknesses are described using weight units of ounces. Although the statement is in ounces only, it is an abbreviation of ounces per square foot. The thickness value can be obtained by the conversion: 1 oz/ft² = 1/64 inch (0.4 mm).
Hence, leather described as 7 to 8 oz is 7/64 to 8/64 inches (2.8 to 3.2 mm) thick.
Found whilst browsing