I will stay on the lookout! 

I have a feeling that the scarcity of these is due to (a) there really wasn't much of a market for them, and (b) they didn't perform all that well. It would be interesting to see a sheet of music actually printed with one of these machines. From the looks of it, the machine could only print on one size staff ruling, and alignment may have been a severe issue. By the time you got all the variables in the correct position and typed a note, you could have written ten notes or more by hand.  That's okay if you're writing the melody for "Happy Birthday To You" but might be a lifetime project if you're composing something like Ravel's "Gaspard de la Nuit" or Schubert's "Piano Sonata/Fantasie Number 18 in G."

I guessing that most of these that were sold were tossed out of garrett windows by frustrated composers. Starving, or not.

My comments were for the Keaton machine. After posting those I had a chance to look over the Music Printing History site. Of all the machines I do think this MusicWriter is much the best. It seems to be flexible enough to do a variety of music writing tasks with spot-on alignment. However, by the company's own admission, it couldn't do circular or arced work. It seems, from looking at the keyboard, that it could print ties between notes. The limitation was in printing slurs. No matter how great the finished product might be from the machine, the composer/musician still have to draw in all the slurs by hand. Presumably with a french curve set or a variable radius gauge.

Also, an interesting thing I noticed on the close-ups of the keyboard was that the Australian machine could print notation of two sizes. One staff size could be printed using one key struck over and over, and the other size needed the hyphen-like character struck with the platen advanced, struck again, etc. At least that's how it looks to have been done. The clefs of the smaller size were printed with a single key, and the larger clefs for the large staff spacing needed two keys.

Don't disparage hand-written music, and photocopied hand-written sheets. You'd never guess it to look at me now after four years of prep school wrestling and lacrosse, but before my voice changed I was a boy soprano in the choir of an highly liturgical Episcopal church. Most of the music from which I sang was specially transcribed and edited and so was handwritten. Especially the soaring descants. Although you are correct in saying that for "camera-ready" musical notation the MusicWriter would have been superior.

I recently purchased one of the converted SG-3 Musicwriters. It is a very interesting machine, but it is a bit laborious to create sheet music on. It is a nice conversion piece, though. It came with additional carbon ribbons, about 15 of them.