Yes, being pitched at "D" is the standard...so it seems.
Could we see the banjo in question? From you brief description of a small banjo with flush frets it might be that you have a small "child" or "ladies" banjo build sometime in the period of 1870s-1900.
How did you arrive and the 1860 year?
Banjos were manufactured in many sizes and one often finds these small sized instruments in the style of the early 1870s. My theory is that they are so prevalent in such good shape because they were not used that much. They were also continuously built and sold through jobbers for about 20 years.
… that is if your banjo is of the variety that I have in mind.
Please, share a picture.
CW- how do you know your banjo is from the 1860s?