Earliest Known Mail from Fort Abercrombie Dakota Territory

This 1858 folded letter sheet represents the earliest known mail item from Fort Abercrombie, Dakota Territory. Notice the "on public service" notation. This was probably carried vial military courier from Fort Abercrombie to the nearest post office, Breckenridge, Minn.

The contents are as follows:

Fort Abercrombie
16 Oct 1858

Mr Barber,
We have no post office here and I herewith ???? the post office bills, are containing certain
letters with the amt collected 45 cts.
G Matten
??
To
The Postmaster
Breckenridge
Minn