Es lo segundo: una placa. Es una expresión que utilizan concretamente en Tucson, y me parece que casi constituye el nombre de un premio. Así que casi me inclinaría a mantener en inglés y añadir la explicación entre paréntesis. No me parece esencial especificar "de cobre".

"There's been a huge outcry from Tucson city employees after the mayor's office announced that there wasn't money in the budget to award "copper letters" to long term city employees in recognition of their service.
Department heads in the city were asked to pick up the tab for the prized plaques instead. [...]
When you come down to the bottom line, it's a copper plaque from the mayor's office. [...]
'I have decided to continue paying for retirement plaques for all city departments out of the mayor's office budget.'"
http://www.tucsonnewsnow.com/story/20617820/controversy-over...

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Note added at 41 mins (2019-01-31 09:39:10 GMT)
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"Friday evening, the University of Arizona honored pioneer heart transplant surgeon Jack Copeland with an endowed chair. [...]
Copeland was supposed to be honored with a Copper Letter from Mayor Bob Walkup, according to the printed program. But Walkup didn't show up; in fact, no one from the city was there. A member of the UA medical staff presented the plaque."
https://www.tucsonweekly.com/tucson/the-skinny/Content?oid=1...

Nótese que en estas fuentes "Copper Letter" aparece con mayúsculas o entre comillas; esto es un indicio de que se trata de un nombre propio. Con que se explique, me parece mejor no traducirlo.

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Note added at 45 mins (2019-01-31 09:43:45 GMT)
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Si buscamos un equivalente en español, a lo mejor correspondería a "hijo/a predilecto/s" (o "adoptivo/a", según el caso), pero personalmente no lo traduciría así. No sé si llega a tanto, si los funcionarios municipales los reciben cuando se jubilan.