Greetings ladies and gentlemen.
I was loaned this cover by a friend and fellow philatelist for assistance in doing some research, especially in using Internet resources.
It seemed to me that the first best thing would be to identify the stamp as best as possible. I believe I have struck it lucky on this front.
I believe this is Plate 3, showing the variety where the right frame line is so close to the design as to appear to be an inner frame line (touching the rectangle at the top of the design and barely missing it at the bottom).
I am not confident, though as Dr. Chase states that 1857 impressions are incredibly clear and beautiful. For the most part this is true except for the mottling in the field around the portrait.
The color definitely looks like the rose brown of the late 1856 to 1857 stamps.
The cover:

The stamp:

The cover is sent as official business to Col. Willis A. Gorman, who at the time was likely still acting as Territorial Governor of Minnesota, a post he filled May 15, 1853, to April 23, 1857. The evidence this far seems to put the cover as being sent in early 1857 at the latest. This is also further backed up by the fact that Fort Snelling and 8000 surrounding acres was sold to private ownership sometime in late 1857 or early 1858.
Regarding Mr. Gorman during the civil war (a few years distant from the mailing of this letter) Wikipedia states:
Quote:
With the secession of several Southern slave states, Gorman offered his services to the army. He was appointed Colonel of the 1st Minnesota Infantry, serving in the First Battle of Bull Run on July 21, 1861. On September 7, 1861, he was appointed brigadier general of volunteers and assigned to command a brigade in the II Corps in Army of the Potomac during the Peninsular Campaign. His troops suffered high casualties during the Battle of Antietam in an ill-fated attack on Confederate positions in the West Woods. Later in the year, he was assigned to command the District of Eastern Arkansas.
Thanks for any assistance.