I was thinking that the tin can in panel 4 might be an anachronism (because even though we play fast and loose with history, I like to know when we are doing so), but it is not. The first tin can was pressed out in England around 1810.
However, a true anachronism with respect to real history is the presence of the gray Confederate uniforms and the Confederate battle flag. These comics take place right after the Battle of Bull Run, and a noted problem during Bull Run was similar uniforms and flags for the two sides.
We wanted to go for the readily identifiable symbols.