Another little known fact (I don't think that this is even covered in either of the mentioned button books), is that the so-called coin buttons, that is the flat non-descript metal buttons, often did not appear that way when new. Many/most of these were originally covered in felt, and some of these were even regulation for the Union Army (i.e. the Model 1858 regulation frock coat for chaplains had flat button covered in black felt). The felt wore away quickly in service, and of course, rotted away when lost on the ground.