I don't know from mill marks on harmonica reeds, but I do appreciate some harmonica work. Here's an ethno-musicological viewpoint:
"The story of the harmonica begins with the Chinese Emperor Nyn-Kwya, who in 3000 B. C. invented a free-reed instrument called the "sheng' (sublime voice) which is considered the forerunner of the modern harmonica.
The sheng was brought to Europe in the 18th Century, where the idea of the free-reed principle was used in the creation of the reed organ, the accordion, the concertina, the saxophone, and the harmonica.
The modern harmonica was invented in 1821 by a German clockmaker named Christian Buschman who put fifteen pitch pipes together to create an odd little instrument. At first harmonicas were produced by clockmakers as a sideline, but in 1857 Matthias Hohner decided to manufacture them on a large scale and went into production in Trossingen, Germany.
The harmonica spread all over Germany, and with the mass emigration of Germans in the latter half of the nineteenth century, all over the world. By the time of the American Civil War, the harmonica was well established in the United States and many soldiers on both sides played them. At first the repertory in this country for harmonica consisted of folksongs, fiddle tunes, marches, hymns and the like, but somewhere along the way it was taken up by the black man, and its potential as a blues instrument came to light.
The origins of blues harp in the South remain obscure in spite of all the musicological research that has been done with blues. W.C. Handy recalled hearing train imitations played on the harmonica as early as the the 1870's, and this was a likely sources of blues harp..."
The harmonica-an introduction by Glenn Weiser
I've not yet seen a photograph of a Civil War participant blowing on a harp.
I do feel that the obliterated person in this photo might have had one, at one time...
The Thirties was another colorful era for Harmonicats. I'll leave you with the Yarraville Mouth Organ Band.