In the area where I grew up, a harmonica was commonly called a harpoon, a french harp, or just a harp. I can't remember for certain, but I believe I remember hearing the term harpoon used by the M.C. in introducing Deford Baily on an old recording of a '40's broadcast of the Grand Ole Opry. Whatever the etymology, the fact is that, in this neck of the woods, a harpoon is a folk name for harmonica.