I think the standard probably had its origins a bit further: My 1919 Westinghouse Catalogue lists control desks that have sections of 16, 20, 24, and 32" wide. The section was covered by a panel that spanned the width of the cabinet - the equipment was intended to be mounted on this panel. From the drawings it looks like the actual space within the sections would be an inch or so less - the specifications are a bit lacking. A 3'x20" rack with side panels ran $817.50!