For a '72, I'm not sure if there ever was an ID plate with accessory/option codes on it. I think those started with 1976 models.
If you have a general idea what the stock color was, you can try matching it up to the chart here:
http://www.cruiserfaq.com/13-00.php
If that's not conclusive, I'd recommend joining the forum at
http://forum.ih8mud.com/ and asking in the 40-series section.