All right: this is direct from the AHS website and explains it all well:
What are the Different Brands? – The American Hanoverian Society i
What are the Different Brands?
H Brand : Prior to 1993, this brand was used to mark North American-foaled HV Main Studbook and Studbook mares and foals. In 2019, AHS members voted to return to this brand. Although Hanoverians are no longer branded in Germany, the HV left the branding decisions to the individual memberships in each country.
H-XX Brand : Effective from 2008 to 2018, AHS foals out of Main Studbook or Studbook mares were branded with the Hanoverian H brand under which appear the last two numerals of the year of birth, which are also the last two digits in the foal’s Universal Equine Life Number (UELN). The addition of the numerals was necessitated by the fact that the AHS used the Hanoverian brand in the United States under exclusive license from the HV and German law required that two digits of the UELN be part of the brand.
H-US Brand : In use in the U.S. from 1993 through 2007, this brand marked foals of Main Studbook and Studbook dams. As part of the Fall 2007 referendum, the AHS membership voted by a two-thirds margin to retire this brand and use the Hanoverian H brand.