Prepared for the Airmail Week celebrations in 1938.
This cover has as its cachet a foil of copper from the mines at Butte, Montana.
The cover was back stamped with the 'signature' of the Butte Postmaster.
Yes! It is Violet.
Additional information is sought regarding the route used from Butte to Kelso.
Possibly Butte - Boise - Spokane?