Overshirts were not uncommon during our period. In fact, given what I have seen in my limited experience with old west reenactors and NCOWS shooters, they may well be underserved in their representation today. Having said that, there perhaps should be some guidelines. The reader of this topic may do well to review a previous discussion that had much good information in it. I decided to begin a new topic due to the amount of images I had.
That previous post had sent me to further research and question my thoughts. Effort trying to find images of overshirts revealed some added information. Without sinking into another strident debate regarding the question: Did the overshirt retains its historical roots and protect the garment beneath or was the garment beneath protecting the overshirt? - Let us consider everything else and limit our discussion from the 1850's thru our period.
Merriam-Webster Internation Dictionary indicates the word overshirt, used to describe a distinct garment, began to be used somewhere between 1795 - 1805.
Brown in his Thoughts on Men's Shirts in America states their evolution began with the smock designed to be used to protect clothing beneath. By the early 19th century the term used in New England was frocks - a garment that became distinct in its own right achieving dress status by our period. At the time the term frock was developing, the term overshirt also developed to represent a garment distinct from the smock or frock.
The frock seemed to evolve into a split front garment, or coat, while the overshirt retained the pullover design of the smock. Brown shows an overshirt recovered from the steamboat Arabia and therefore with an 1856 date. It is square cut (shirts by then were starting to be more fitted or French cut), has a collar, and a small shield front with brass buttons. It has a large pocket sewn onto the shirt and quite low on it. For this reason, Brown suggests the shirt might have been designed to wear outside of the pants, the shirt is but 28 inches long (much shorter than a smock of the same period) at a time when shirts seemed also to be longer.
As time progresses, the sewn on pocket seems to be a common trait on overshirts. Although not unseen on shirts of the CW period and later, they seem more common on overshirts. The overshirt, at least during the CW and the beginning of our period, seems to be less fitted, and square cut. This changed with time and with the the popularity of firemans shirts and later bicycle or laced front shirts (a subset of overshirts).
The issue of whether overshirts should be worn outside or inside the pants should be addressed. With Browns comments and the dashing image of a Missouri guerrilla aside, I was hard pressed to find any image of one worn outside the pants during our period. At this point I would suggest that for our period, an overshirt seems to have always been worn inside the pants, with much longer smocks still worn on the outside and quite different in appearance.
With that let us consider some images, most of which you are probably familiar:
Below is probably the earliest image I have purportedly either just before or during the 49 gold rush period. Interestingly Brown includes this on the page discussing smocks. Note no pocket, square cut and drop shoulder, although a bit difficult to see appears tucked into the pants even at this early date.
About the same time period or slightly later, these are identified as two Californios. The weapon would suggest 1849 or later. Note a fairly blousey overshirt now tucked into the pants.
A common picture we all have seen circa 1850's. Note the fella in the middle with the rather colorful overshirt, tucked into his pants. Beginning to appear somewhat more fitted.
From Echos of Glory, a confederate cavalryman, I believe I can identify pockets on the overshirt but note - tucked in.
Also from Echos of glory, note the ribbon trim, pocket and tucked into pants.
And of course Jesse James, different from other images is the cut about the neck and decorative spots, note the large pocket. It is difficult to clearly state inside or outside pants.
All these images predate our period by a decade or more. But note common elements, pockets, somewhat bloussy, shirt or at least collar exposed. Use of cravat with shirt.