Description of Problem:

Looking at /usr/share/fonts/default/Type1/fonts.scale one can see there:

n022003l.pfb -URW-Nimbus Mono L-medium-r-normal--0-0-0-0-p-0-iso8859-1
n022003l.pfb -URW-Nimbus Mono L-medium-r-normal--0-0-0-0-p-0-iso8859-2
n022004l.pfb -URW-Nimbus Mono L-bold-r-normal--0-0-0-0-p-0-iso8859-1
n022004l.pfb -URW-Nimbus Mono L-bold-r-normal--0-0-0-0-p-0-iso8859-2
n022023l.pfb -URW-Nimbus Mono L-medium-o-normal--0-0-0-0-p-0-iso8859-1
n022023l.pfb -URW-Nimbus Mono L-medium-o-normal--0-0-0-0-p-0-iso8859-2
n022024l.pfb -URW-Nimbus Mono L-bold-o-normal--0-0-0-0-p-0-iso8859-1
n022024l.pfb -URW-Nimbus Mono L-bold-o-normal--0-0-0-0-p-0-iso8859-2
n022003l.pfb -URW-Courier-medium-r-normal--0-0-0-0-p-0-iso8859-1
n022003l.pfb -URW-Courier-medium-r-normal--0-0-0-0-p-0-iso8859-2
n022004l.pfb -URW-Courier-bold-r-normal--0-0-0-0-p-0-iso8859-1
n022004l.pfb -URW-Courier-bold-r-normal--0-0-0-0-p-0-iso8859-2
n022023l.pfb -URW-Courier-medium-o-normal--0-0-0-0-p-0-iso8859-1
n022023l.pfb -URW-Courier-medium-o-normal--0-0-0-0-p-0-iso8859-2
n022024l.pfb -URW-Courier-bold-o-normal--0-0-0-0-p-0-iso8859-1
n022024l.pfb -URW-Courier-bold-o-normal--0-0-0-0-p-0-iso8859-2

In other words we have here an example of a proportional Courier font
which is rather unusual. :-)  Nimbus Mono fonts are remarkably similar
but this is not so surprising if one will notice that these are really
the same files with obvious implications for glyphs.

In reality all of the above are truly monospaced fonts which can be
trivially seen if they are used as text fonts in an editor like emacs
or vi.  I know that all pfb files above have
/isFixedPitch false def
but this seems to be an editing error in URW fonts and there is
hardly a reason to propagate it further. All '-p-' above should
be '-m-'.  A half-hearted attempt to fix that with the help of
'fonts.alias', done for Courier, does not seem to work if 
"Nimbus Mono L" names are used (blanks in names?).  Changing
fonts.scale, and fonts.dir, without touching fonts works like it should.

BTW - a name like "Mono L" is not the luckiest one; after loading
in gfontsel a lower case sans-serif fonts is used to display names
and "Mono L" does not differ from "mono 1" ('one' not 'ell').