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You can get a great downloadable list of ISO 3166 country codes from the ISO and this can help you create a drop-down list with matching values for your application. Either leave the configuration alone, and just let Outlook users see the Default Global Address List; or else, take the time to study, plan, and test before you implement custom Address Lists.
The l DAPDisplay Name for the Country-Name attribute is a lowercase c.
The confusing part is that the value of the Text-Country attribute (which has the l DAPDisplay Name of co) matches the text displayed in the Country/region drop-down list box.
So, if you're trying to use ADSI Edit to reverse-engineer this attribute, you can easily conclude that Text-Country is the attribute you need to set to modify a user's Country/region setting. If you're trying to write a script that duplicates the behavior of manually setting the Country/region option in Active Directory Users and Computers, you need to make sure you update all three attributes.
If you were trying to use the Active Directory Users and Computers snap-in and ADSI Edit to figure out which attributes to set—and it sounds like you were—I'm not surprised you overlooked the Country-Name attribute.
If it's any comfort, you're not the first person to ask me this question. When you use Active Directory Users and Computers to change a user's Country/region setting, not one but three of the user's attributes are modified in AD: Country-Name, Text-Country, and Country-Code. You use the Country-Name attribute to set a user's Country/region setting on the user's Address property page.
Their pre-Windows 2000 AD logon name was changed as well to reflect their new name.