I'm having difficulty determining what the original game is supposed to look like. I have attached two images, one which has "keep aspect ratio" from the GOG graphics options setup turned on, and one that doesn't.

The image that has it on is 4:3 (1.33333) aspect ratio, but looks like it may be stretched vertically (circles are ovals, squares are rectangles). The other image is 16:10 (1.6) aspect ratio, and doesn't seem to be stretched, however 16:10 is a very odd aspect ratio for the era in which the game was released since widescreen monitors didn't exist (to the best of my knowledge)

Which screen shows what the game actually looked like in 1992 when the game was released?

I'm pretty sure it's 4:3. Since there weren't any widescreen monitors back then, there weren't any widescreen resolutions either. Most DOS games at the time used a 320x200 resolution.

Here is a bit more technical information about the game if you are interested:

[url=https://pcgamingwiki.com/wiki/Ultima_Underworld:_The_Stygian_Abyss]https://pcgamingwiki.com/wiki/Ultima_Underworld:_The_Stygian_Abyss[/url]

I admit that even though it does look a bit natural in widescreen, I'd stick with the 4:3 aspect ratio. It's just the way games were meant to be played in those days.

Part of my confusion stems from the fact that old school monitors like the ones UU was designed for did not have square pixels like our current ones do.

I'm not sure what the original resolution for UU was, but it was probably 320x200 as mentioned above. However, there were several other modes available on VGA adapters. Some officially supported, some seemed a bit hack(ish) and not supported on all cards...

320x200 (most common earlier VGA DOS games)
320x240 (some later VGA DOS games I think referred to as QVGA)
640x480 (VGA mode, but usually SVGA with 8 bit color or above)
800x600 SVGA
1024x768 SVGA
1152x864
1280x1024
etc.

I've also seen some other oddball ones like:
320x400 (I believe this was only used for scene demos and called Mode X)
400x300 (unless this one was Mode X)
512x384 (I can't remember where I saw this, but it was probably some overscanned mode or something...)
640x400

Anyway, if you're seeing it stretched it may be because of the difference between 320x200 and 320x240 modes. Both are fairly common, and I've seen them used interchangeably in DOSBox before, so maybe that's what's happening.

I just took a look at the game on my old 486 laptop and it looks like in your _keepaspecton.pgn.

I did some additional digging and the game certainly ran at 320x200. However, I forgot that most old CRT monitors let you move and resize the screen manually via buttons on the chassis. The closest 4:3 VGA resolution would be 320x240, which means the game is missing 40 rows of pixels. Most people probably didn't bother to resize and let the image stretch vertically, which would result in the same look as having "keep aspect" checked in the GOG setup. If you took the time to resize your screen you'd get the other look, which is most likely correct based on the intended resolution and screens from the game's manual.

Some sources:
http://nerdlypleasures.blogspot.com/2013/10/320x200-resolution-of-choice-for-ibm-pc.html
http://mocagh.org/loadpage.php?getgame=uuw

ninja'd