Hmmm...
While I was considering editing the binary font file (helvB12.pcf.gz),
I noticed a different font file in the same location:
/usr/X11R6/lib/X11/fonts/75dpi/helvB12-ISO8859-1.pcf.gz
After some inspection, it would seem that many fonts default to
iso10646, but also are available in a iso8859 versions.
What is the "Debian way" to give preference to the iso8859 versions?
I found that simply deleting the iso10646 version (helvB12.pcf.gz)
and then doing a update-fonts-dir does work.
BUT -- I think as soon as package xfonts-75dpi is upgraded, it will
undo the fix...