the meta name="og:country-name" content="?"
Normally I would use a 2 letter ISO such as GB or US, but I have seen there is a 3 letter such as GBR or USA exist published by the International Organization for Standardization (ISO).
Upon doing a search, I now see others under ISO 3166-2. These allow for provinces, such as GB-ENG for England, there is also one for Great Britain "GB-GBN" or UK "GB-UKM". I thought there might have been a GBR !! lol
For now, I'm also adding the two later ones. Will they be read by bots or other devices? Also, going by OGP open graph website, they use an underscore, unlike the lang= versions.