I looked through some old Scientific American reprints and found vertical dropouts on a bike in an 1899 advertisement. They were used with an eccentric bottom bracket for chain adjustment. The more common bikes seem to have rear-facing frame ends, like track bikes or roadsters (but the pictures aren't very sharp) The fork on this bike had holes for the axle; not slots. (Were there lawyers in 1899??)
One of the articles mentions that the technology was so perfected that perfectly good bikes were available weighing only 22 pounds (and that's with little or no aluminum).
David Bean
Arlington, MA