Two school teachers married in Earlville, Iowa in 1879
The missing marriage record for Herbert Stetson and Harriet Smith turned up when I searched in Family Search (for some reason, Ancestry doesn't have it). The marriage was recorded in Earlville, Delaware County, Iowa on June 17, 1879. Earlville. of course, is the town where they would live for the next 25 years while raising six children.
In the 1880 census, exactly one year after their marriage, Herbert and Hattie Stetson are listed as boarders in the Van Wagenen household on Manchester Street in Earlville. Herbert's occupation is "Principal of School" and Harriet (Hattie) is listed as "School Teacher."
So this new information tells us where they married, and it certainly fits the timeline. However, we still don't know where they met. Did they arrive in Earlville already a couple, or did they meet in the small town after each arriving separately?
The latter could be the more likely scenario if they had both been hired to staff a newly opened school in, say, September 1878. They boarded separately, of course. But plausibly they became close as they worked together that first school year.
They married in June and then they lived together in the Van Wagenen home for the next school year and maybe longer. Later they made their own home by the time their first child, Ralph Haskell Stetson, was born in August 1883.
This interpretation fits the facts better than alternative scenarios where Herbert and Hattie would have met either in Ohio or Maine. So, pending any new information, let's assume that Herbert and Hattie met in Earlville where they each had come separately to take up their new jobs.
The postcard shows Earlville High School in 1910, by which time the Stetson family had moved on to Montana.