Goss:
I talked about your issue with a contact I have at Honda in Marysville, who used to work in the (now defunct) Motorcycle operation.
He confirmed that it is unlikely that official Honda locations anywhere have documents about older motorcycles that will provide info about manufacturing locations for specific models and tie them to VIN-numbers that do not include the plant codes (for the CBX, that started in 1981). In your specific case, he theorizes that, because of the bike was imported from the U.S., that this was used as the "country of origin" by Honda on the document.
Another - less likely - possibility is, that, as CBXes were produced in the U.S. from September 1979 on, someone made an honest mistake and thought your 1979 was part of this.
In any case, it is really a shame that official manufacturers documents and statements can't be more correct.
But I am sure, you will enjoy your CBX-Z to the fullest anyway