This usage of a Parcel Post stamp to cover the WW I war rate may be unique.
It was mailed after the postage rate increase in early November, 1917.
Use of this stamp is a ’late usage’, since it was issued in 1912-13. The item was probably mailed by a soldier at Camp Custer, Michigan, which was a major training facility for the Army in WW I.
The stamp is cancelled by a Universal machine of the Custer Branch, Battle Creek, Michigan.
Note that Parcel Post stamps were legal to use on first-class letters in mid-1913, but such usages after 1914 are very uncommon.
See this webpage on Parcel Post stamps, for more information.
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