How many onions / lbs does a contract represent?

A contract represents a set of (quantity, month) tuples, where quantity is an integer greater than zero; that number of yellow onions will be delivered in the corresponding month to the current holder of the contract. The mass of each onion is unspecified, as is the mean and variance of the underlying onion-mass distribution from which they are drawn.

Pricing is per onion. You could order some and weigh them, and report back.