I found this postcard being offered for sale on Ebay and thought the image would have a lot of interest to postal history buffs, so I decided to post it:
I believe that's a "U.S.M." (US Mail) lettered in the area of the coach (right under the driver).
I also found it interesting that the general store shown doubled as the post office and the name on it is F.L. Scott (Frederic L. Scott). Not surprisingly, Mr. Scott, the proprietor of the store, also served as Farmington's Postmaster back in the day. In fact, I even found this image of him:
I found it an interesting tie-in to a classic postcard which is also a piece of postal history.