I think I've got a Sharp pen plotter that's
the same - little tiny pens and a
paper roll (although IIRC you can feed it cut sheets as well). I've yet to
find anywhere that sells the pens, but apparently Radioshack used to stock
IIRC, the mecahnsim was made by Alps, and there are at least 2 sizes.
There was a small one, about 2" wide used in the Sharp printer for the
PC1500 calculator (sold in a slightly modified form as the PC2 by
Radioshack). And a larger one, used in the Atari printer that started
this thread, the Commodore 1520, Radio Shack CGP115, Oric printer ,
etc, etc, etc. Both units use the same pens.
There was also a standard controller IC (it was a mask-programmed
microcontroller, I forget which one, but it was something standard --
maybe 6805-like), which was used in some of the desktop printers. It had
Centronics and 600 baud serial inputs.
I have the Alps service documentation for this mechanism, but it doesn't
show the motor pinion separately from the motor.
I also haev an electronic 'typewriter' which uses a similar mechanism,
but A4 width. It has a Centronics input, and responds to the same
commands as the smaller units.
Nor do I know of a source of pens. Radio Shack used to sell them years
ago, not surpisingly. IIRC, there were 2 packs, a set of 3 black pens,
and a set of 1 each of red, blue, green. The 'typewriter' I mentioned
also can take a pen filled with Tipp-ex like correction fluid to use to
delect characters (I am not joking!).
 The Oric printer came in 2 colour schemes, white to match the
original Oric 1, and orange to match the Atmos. Internally they are
identical. Talking of Oric add-ons, does anyone know anything about the
3" disk drives for the Oric. I have a pair which have been messed about
with by the previosu owner (I think he had 2 'master' units, with the
internal controller board and was trying to turn on into a slave). One
day I will sort them out.