We (Apple) tested the HP DeskWriter driver version 3.9 (both serial
and AppleTalk) on the Centris 660AV and 840AV. As you stated, the
660AV required the gray scale option (checkbox) to be selected in
order to print. The Centris 840AV had no difficulty printing.
As a workaround with the 3.9 driver, if you need the DeskWriter to
print on the Centris 660AV computer, you will have to select the
gray scale checkbox in the print dialog box.
We spoke with Hewlett Packard. They do not have a solution at this
time.