Sorry, Greyhoundman. You are of course quite right about the short or US quart -- brain fade on my part -- but historically the short gallon was known as wine measure. A number of measures were not standardized when the colonies rebelled, and this was one of them. There's also the short ton, variously 2000 lb and 2200 lb, instead of the full Imperial ton, 2240 lb (20x 1 cwt @ 112 lb -- and don't ask me why that's a hundredweight...)
Intriguingly, too, some words diverged, such as 'haberdashery' which embraced both the US and English meanings until about 1760-1780.
But surely a 'fifth' (? of a 128-fl-oz gallon, = 25,6 fl oz) is a common term for a bottle of spririts?
Cheers,
Roger