We have a product called "SERIAL" which then enables us to enter the serial number of machines in the description on the invoice. When customers sell machines, change business names etc we need to be able to search for the serial number.
We also have a product called "IND PARTS" and in the description on the invoice we type the part number. Being able to search by part number in invoicing would allow us to find specific customers.
We do not use Sage for stock control hence that is why we use generic product codes ("SERIAL" and "IND PARTS" are just 2 examples).
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When invoices are posted to the accounts, only the Description line is posted - the details line is not. As long as you can get the serial/part number into the description line (with a suitable header, eg "Ser No: " or "Part No: ") you can search for it in the transactions module, which will then lead you to the customer account through which it was invoiced & posted.