Yes, it was definitely manufactured for the Civilian clothing-market. But of course, some soldiers (especially Militia soldiers) did put "plain" flat-buttons on their uniforms.
I do not recall seeing an explanation in any of the various button-books for the eagle on some "flat-button" backmarks. So, the following is just my "impression" ...based on my several decades of Historical reading, and button-digging, and button-dealing.
Prior to the American Revolution, the colonies imported nearly all manufactured goods from England, with a much smaller amount coming from France and other European countries. British-made buttons often had the (distinctive) British crown mark. After the Revolution, manufacturing in America took decades to "get off the ground." So, during those decades, even though Independence had been achieved, the American public still needed to import most of the manufactured goods they needed in daily life. Even, metal buttons. This is why so many buttons from 1790s-to-early-1800s US sites have backmarks with British maker backmarks and spelling (such as "colour"). The War of 1812 caused increased resentment about needing to import British-made products. Then, at long last, in the late 1820s and 1830s, American button-making firms became solidly established, and supplies of needed metals such as copper and brass became more accessible to the young American button-making Industry. To distinguish their US-made products from very similar-looking British imports, US button-makers put the distinctive American eagle in their backmarks.
That's my theory. In summary... if you see "Treble Gilt" (or etc.) with a crown, the button is British-made ...and "Treble Gilt" (or etc.) with an eagle is US-made. (I do not recall ever seeing an American eagle in a backmark with a British word (like "colour," or "London".)
