changing GTK print dialog defaults?

["John D. Baker" <jdbaker%consolidated.net@localhost>]

My searches have so-far turned up nothing useful regarding where the
default values in the print dialog offered by GTK-based applications
(Firefox [when selecting "Print using system print dialog"], Gnumeric,
Evince, etc.) come from and how they can be changed or otherwise made
to persist.

Chiefly, the default paper size always comes up as "A4", even after
previously selecting "US Letter" in a previous use of the print dialog.
If I forget to change it, then either the print job is silently discarded,
held in the printer's internal queue until A4 paper is loaded, or caused
to print from the tray holding "US Legal" size paper--frequently printing
only a portion of the original page thereon.

"/usr/pkg/etc/papersize" contains: "Letter", as per my setting of
"PAPERSIZE" in "/etc/mk.conf".

Secondly, although it rightly defaults to "Print to LPR", I want a
different "Command Line:" default (current default is 'lpr') since I have
to convert (chiefly) Firefox's output from PDF to PostScript and prefer
it print duplex.  I have local scripts which do this and pass the output
to 'lpr' which simply forwards it to my duplex-capable PostScript-speaking
printer.

I do not use a GTK-based "desktop environment", just Fvwm (2, if it
matters).  Environment variables like "XDG_foo" do not exist in my
setup, so I don't know what those values would be.

I do not use CUPS, so nothing related to PPD files has any relevance.

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