吋 is specifically used for the English inch. Do not confuse with 寸, the traditional Chinese unit of length (about 3.33 cm). In Taiwan and Hong Kong usage, 吋 is common for screen sizes.
口 here functions as a marker for foreign-borrowed terms, not literal mouth. Many transliterated units take a 口 prefix in older texts (吗哩咪). It signals to readers this is a phonetic loan rather than a native concept.
寸 supplies the sound and a parallel meaning - it is the native Chinese inch, here pressed into service to render the English inch. The combined character means imported inch versus 寸's domestic measurement, clarified by the mouth marker.
