Oh a dozen predates the imperial system by a long way. From wiki because I'm a lazy...
The dozen may be one of the earliest primitive integer groupings, perhaps because there are approximately a dozen cycles of the Moon, or months, in a cycle of the Sun, or year. Twelve is convenient because it has a maximal number of divisors among the numbers up to its double, a property only true of 1, 2, 6, 12, 60, 360, and 2520.[1]
The use of twelve as a base number, known as the duodecimal system (also as dozenal), originated in Mesopotamia (see also sexagesimal). Twelve dozen (122 = 144) are known as a gross; and twelve gross (123 = 1,728, the duodecimal 1,000) are called a great gross, a term most often used when shipping or buying items in bulk. A great hundred, also known as a small gross, is 120 or ten dozen. Dozen may also be used to express a moderately large quantity as in "several dozen" (e.g., dozens of people came to the party).[2]
Varying by country, some products are packaged or sold by the dozen, often foodstuff (a dozen eggs).
Barry's.