this image shows pretty clearly what is part of the ISO code that is required:

http://en.wikipedia.org/wiki/File:Containernumber.jpg

the size and type codes aren't used in the check digit, are not required, and are usually displayed on a separate line:

http://www.containerking.co.uk/buy-cabins/images/Shipping%20...

anyway, back to the original question, you can't have two containers with the same owner code, same container ID and made unique through the size and type codes

the problem is solved because the large shipping firms (ie. the 5 or 6 that control the industry) each have dozens of manufacturing codes. see my answer in the other part of this thread

Hmm, I maintained software dealing with shipping containers for the trucking industry and occasionally there where identical boxes that had different size and type codes. Granted they could have been data entry errors, but the software used the size and type codes to uniquely identify things.