Hewlett-Packard calculators have a 10 character serial number which can be used to find the date and country of manufacture. It may be stamped in the plastic or on a sticker (possibly inside the battery compartment). It is known that in practice the HP factories often left the week part unchanged for a while so you cannot rely on the week number part of the serial, it should be considered as a rough guide only. As later HP serial number formats repeat you may get more than one date returned. If this is the case you can look up the specific HP calculator's manufacture date range to find out which years are valid.
HP calculator serial number :
Click herefor an idea of what this HP calculator may be worth (via previously sold items on eBay)