Just found this, thought I'd pass along -
This ad from the March 1905 issue of "The Railroad Telegrapher" for a company that specializes in Telegraphy equipment (as far as I can tell) mentions mill
typewriters. Also, see if you can find "73" on it: http://vintagevibroplex.com/first-ad.jpg
It is said to be the first ad that mentions the Vibroplex key, then the brand new alternative to the pricey (and power hungry?) Autoplex.