After finding enough screenshots/video reviews, I am beginning to think that from a hardware perspective the S and L variants are identical (including supported bands), whereas the K is only slightly different from S/L.
Any images of the back cover off on the K models (Olleh) will show "F400 0425 K" indicating the hardware is the "K" variant. (If you watch the review video http://www.androidcentral.com/lg-g3-review, you will notice this info up at the top on the backside, right above the camera.) UPDATE: Here is another link with an image http://arstechnica.com/gadgets/2014/...ixels/#image-4
On the S/L (SK Telecom, LG U+) models, they say "F400 0423 S/L" on the SK Telecom model ("S" variant), and "F400 0425 S/L" on the LG U+ model ("L" variant). You can find images of the "S" model with engadget's review (http://www.engadget.com/gallery/lg-g...!slide=2676963), and images of the "L" model with ubreakifix's F400L teardown (http://www.ubreakifix.com/blog/wp-co...ubif-g3-21.jpg).
(Thanks and kudos to the review sites and teardown site for the images.)
As an owner of the F400S, I can tell you that all 3 variants will run on AT&T LTE (as a lot of our LTE is on Band 17 - 700 MHz spectrum). I was able to load my F400S with the TOT and DLL files of all 3 variants (currently waiting on getting it back to the "S" variant once the DLL is posted), and all 3 variants will connect to AT&T with no issues (for those of you that plan on importing like I did).
So for those of you who managed to brick your new G3, as long as you can ADB into it, you can edit your build.prop file with the corresponding korean variant model info, and load any stock image. As @wolfgart has posted a great thread with the TOTs and DLLs, I recommend swinging through there for more info. If you have a K variant, load the K TOT and DLL, and you are fine.
For those of us with S/L variants--with the above info and enough tinkering in the hidden menus of all 3 variants, I can tell you that from a hardware perspective these two models are nearly identical. From a modem/supported frequencies perspective, the S and L variants support the exact same frequencies--meaning, if you bricked your S or L version, you can restore it with an S or L TOT and DLL (after editing build.prop if needed) and you will still get all of your original hardware supported bands/frequencies. This is critical for those of you using LTE Band 5 to connect to your data... (should not be the case for any US AT&T users...).
UPDATE: in the 'Android Development' section, wolfgart's thread with the TOTs and DLLs have been updated to now include the files for all 3 models. So unless you know what you are doing with a build.prop edit, probably best to stick with what came with the phones for those who are new to this stuff.
The main difference between the S/L and K phones in frequency/band support is that LTE Band 8 is supported on the K model only, whereas you will get LTE Band 5 support on the S/L models. All other bands and frequencies (WCDMA/GSM) are the same across all 3 models. All 3 also support LTE Band 17, and will connect to AT&T in the US.
Took me a while to figure this out, but I had a long weekend