Here's a link to the UPU Conventions on the UPU web site:
http://www.upu.int/en/the-upu/acts-...volumes.htmlThe
Conventions document contains the Universal Postal Convention and a lot of supporting information on both letter post and parcel post.
For example, here from page 8 is the wording regarding from Article 6 of the convention on the identification of a country on its stamps:
3 Postage stamps comprise:
3.1 the name of the member country or territory of issue, in roman letters ***,
or, if the UPU International Bureau is so requested by the member
country or territory of issue, the abbreviation or initials officially representing
- the member country or territory of issue, in accordance with
- the conditions laid down in the Regulations;
3.2 the face value, expressed:
3.2.1 in principle, in the official currency of the country or territory of issue,
or as a letter or symbol;
3.2.2 through other identifying characteristics.with the following footnote (my *** symbol above):
An exception shall be granted to the United Kingdom, the country which invented the postage stamp.
There are regulations and guidance about printed postal impressions on the next few pages including examples on page 14 with two-dimensional codes such as in the original post.
This is not a QR code but makes use of UPU's own S28 technical standard.
I do wonder as hy-brasil has suggested whether this includes a country code.
The coding structure in the S28 standard is defined in a separate document.