You can download public-domain data about United States of America locations at
and these data could be useful for your GOV database.
The competing credit card companies have worked out a way to avoid trouble:
America Express always starts with 3
Visa always starts with 4
MasterCard always starts with 5
Discover always starts with 6
The GEDCOM 5.5EL standard could allow for public-domain databases usable by any genealogy software:
0 @P_GOV_1234567@ _LOC
...says that the @P@ tag is from the GOV database. Maybe FamilySearch might introduce a dataset
0 @P_FS_1234567@ _LOC
and my own GEDCOMINDEX.COM which uses my own place codes ("GEDCOM Index Place Codes")
0 @P_GIPC_DEBY2249@ _LOC
...see my page at http://gedcomindex.com/Pages/DE/DEBY2249.html
Nashua, New Hampshire, U.S.A.