Hi Chris,

Thank you for contacting Mearto with your appraisal inquiry.
Enameled porcelain signs were far more common, for the USPS and other businesses; aluminum was a revolutionary material for signage and is highly collectible. With few existing examples, most auction houses that specialize in signs - particularly second-half 20th Century signs, would probably value this at $3,000-5,000, which is both low enough to generate interest and high enough to acknowledge its rarity and potential value. If I saw this at auction, I would not be surprised if it sold for more , possibly even much more, but putting too high an estimate is certain to dissuade or deter potential buyers.
Based on the photos and information provided, and subject to examination, this is:

A rare vintage USPS aluminum prototype sign
circa 1960s-70s
The rectangular sign with rounded corners with embossed writing "UNITED STATES POSTAL SERVICE / US MAIL" and centering an eagle in profile.
36 by 34 by 2 inches
CONDITION: This sign appears to be in excellent condition
PROVENANCE: Inherited from a
$3,000-5,000*
*represents a fair-market value for auction purposes; retail or asking price may vary.

Please let us know if you have additional items to appraise, or questions/concerns, and thank you again for using Mearto.
~ Delia