Here is a useful reference from the World Intellectual Property Organization (WIPO) ‘Handbook on Industrial Property Information and Documentation’.
This Recommended Standard provides two-letter alphabetic codes which, for the purpose of improving the access to industrial property information, represent, in particular, the names of States, other entities and intergovernmental organizations the legislation of which provides for the protection of industrial property rights or which organizations are acting in the framework of a treaty in the field of industrial property.