Duncan, approx. how many widgets might you be dealing with?
There is the option in QBooks that you could make use of another field in the invoice which would permit you to "search" for a specific serial number, but this would only give you information on the "sold" serial numbered item.

Can you be more specific for your "need" for the serial number, could you work on the basis that "if not sold - should be in stock?"

Perhaps a very simple access database, not linked to QBooks will be your most easily controlled option.

Yvonne