As you've discovered, your EC-100 is the Epiphone equivalent of the Gibson C-1 - had a friend who owned the latter when I was in college and it was a nice little guitar, sweet-sounding and very comfortable to play...
Similarly, your EC-300 is the counterpart of the Gibson C-4 as I suspected - here's a photo of the latter showing the same quasi-mid-1800's headstock which, as you state, was not used on any other Kalamazoo instrument (note also the slightly different rosette on the Gibson - possibly a matter of availability, as the upper-end classicals were notoriously slow sellers):
