An Honorary Placronym seems to be an item unique to the post-Scratch universe, and is first seen . On a child's thirteenth birthday, they are finally given a name, which is then engraved on a plaque similar in shape to the original Naming box seen .
Alternately, naming boxes could turn into placronyms when engraved. Though, a naming box seems bigger than a placronym.
Ultimately it is likely that, like its "predecessor" the naming box, it is a narrative framing device, since John signed letters with his name before his 13th birthday. It is, however, also possible that kids are given names when they're born, as is typically the case, and it is merely a post-scratch custom to have their names engraved on an Honorary Placronym when they turn thirteen.