I,v got a photo here of a vertical, slide valve Progress engine dating from the early 1930s, all I did to it after this was to spruce it up, and mount it on a better looking lump of wood.
Originally there were four Progress engines, a slide valve horizontal, and a horizontal oscillator. Then a vertical Slide valve, and a vertical oscillator. The idea was that the beginner would start with the oscillator/wobbler as it was simpler to build, and cheaper.
Ian S C