I’m not trying to start an argument, but in all the time I bought and sold used shipping containers not one invoice, incoming or outgoing, contained a serial number. We bought from the shipping companies and sold to individuals. If there was any specific container ID on the invoice, it was the stock number assigned by the previous owner. Most of the time we bought and were invoiced for “X number of Y size containers in Z condition”. Semi-trailer invoices, OTOH, always had each vehicle individually listed with the VIN - which we checked closely against the invoice, delivery ticket and title before accepting delivery.
IOW, when buying directly from the manufacturer (as I presume your company does), a serial number may be supplied on the paperwork. Once it hits the secondary market, however, traceability ends.