These could refer simply to LOT #'s. Most Darjeeling is sold in multitudes of lots...the lot # is usually stamped onto the large wooden crates. This is how they track the tea's origin, harvest info, etc of the tea.
Expiration dates are American designations required by the FDA on packaged tea products...so unless it is on a box of tea bought at the grocery store or a vendor who also complies with this obscure regulation...my bet is that it is a lot #...unless an importer is attempting to comply with the regulation.
I know Upton applies these lot #'s at times.