Posting the thing that started this project, because it is the oldest open question I had and it took until now to answer it properly.

I grew up on one of these. From 1989 to about 1995 a Kodak-badged Diconix was the only printer in the house: school papers, cards, Print Shop banners. Every single thing with a graphic in it came out at roughly two thirds size. Not cropped, not clipped, just small, floating in a sea of white margin. Plain text was fine. Anything else was two thirds, and the aspect always looked faintly squeezed on top of that.

I was nine. I assumed that was just how printers were.

The answer is in HP's own service manual for the ThinkJet 2225, which is the same print engine under a different badge. This family lays its dots on a fixed grid:

96 or 192 dots per inch horizontal. 96 dots per inch vertical.

Epson's FX graphics standard assumes something else entirely: 60, 120 or 240 dpi horizontal depending on which density command the software sends, and 72 dpi vertical, because an Epson 9-pin fires eight pins across a ninth of an inch. The printer faithfully places every dot it is handed, on its own grid, and nothing anywhere in the chain rescales.

• Vertical: 72 requested into 96 actual = 3/4 height. Always, no exceptions.
• Horizontal, double density, which was the common software choice: 120 into 192 = 5/8 width.

Two different fractions on two axes. That is the two-thirds page, and that is also why the aspect looked wrong rather than merely small.

DiKon does not emulate anything. It takes 300 dpi PCL3 raster and does the interleave to the head's native grid on-chip, so the geometry is correct by construction instead of by accident. Which means the specific annoyance that made me pick this project up is now a thing the firmware handles, thirty years after it first bothered me.

Side-by-side prints of the same file, stock Epson mode versus DiKon, to follow.