There are a bunch of negatives of probable Civil War veterans taken between the 1870's and 1910's in C.M. Bell's Washington DC studio which are unidentified at LOC. He seems to have became something of the premiere photographer in Washington DC after the decline of Mathew Brady's business. More about Bell here: https://blogs.loc.gov/picturethis/2017/04/c-m-bell-studio-collection-newly-digitized-portraits/
Here is the first one I've come across:
Source.
Here is the first one I've come across:
Source.