[Earliest known letter from the Denver gold mining region, 1858], letter datelined “South Platte Near the Rocky Mountains” on October 28, 1858, prepaid 3¢ (#26), letter from "Pinky" Stout to Cherry Fork, Ohio mentions, “An officer of the army at Fort Kearny who came out with us returns tomorrow,” entered the mails with December 4 postmark of Pacific City, Iowa, further details see page here

(MWE, Figure 12-3) ex Baughman, Risvold and Walske (Image)