It is indeed a Scott US #638(1927Jul27). It is a color-changeling.
It is usually considered a damaged stamp, but it's a nice conversation piece. If you want an explanation, read the next paragraph.
The brownish stamp suffered prolonged exposure to sulfur-containing pollutants, causing the near-surface pigment to turn black as the sulfur became incorporated into the pigment. This happens to certain orange and yellow stamps. The underlying pigment is still orange. So technically, it is possible to convert the blackish sulfur compound into a semi-transparent layer, thereby revealing the underlying undamaged color of the stamp. In fact, there are quite a few collectors and dealers who practice this. There is a debate regarding whether or not this chemical treatment of the stamp should be considered an "altered" stamp. Contrary to popular understanding, the blackish layer is not actually being removed, only converted to a semi-transparent layer. As far as I know, there is no practical way to physically remove the blackish layer without causing stamp damage. Also, that thin sulfur-contaminated surface layer is NOT recoverable (i.e., cannot be converted back to the original color). The most you can do is expose/reveal the remaining underlying orange pigment.
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