Joining the party late, not sure if you ever found the transmission model numbers or anything or if you're even looking still. I can't say exactly what the numbers are, but I've been working inside both style transmissions and will try to share what I've found:

The retainer with the countersunk bolt holes looks like the one I have out of a Starion. Being from a Starion, mine has a wideblock (2.6L) bellhousing, and doesn't fit the 2.0L in my truck. It should have a 23mm input shaft bearing, tapered rollers on both ends of the countershaft, and a separate reverse gear as part of the 5/R assembly. Reverse gear is helical cut on this transmission. Additionally, the bearing surfaces on the countershaft have a larger diameter for the tapered roller bearings.

The flat retainer looks like the ones out of my KM-132's for the 2.0L in my truck. The countershafts both use ball bearings, not tapered rollers. Therefore the countershaft bearing surfaces have a smaller diameter. Reverse gear is built into the ring on the 5/R Hub, there is no independent reverse gear, and therefore no reverse synchro. Additionally, reverse gear is straight cut on those transmissions. The input bearing is only 21mm thick. The lack of a helical cut reverse may cause issues depending how you mix and match parts, that larger reverse idler is matched to only the helical cut reverse gear.

As you said, there are many variations of the KM-132 transmission. They all use very similar but slightly different parts, which can make it very confusing. If you are interested, I can share a couple manuals I've found that show exploded views of each variant, which can help identify which sub-models you have. I'm no expert, and have been working through many of the same issues myself the last couple of years. One of these days I'm going to do a full write-up of what I've found, but that hasn't happened yet.

As mentioned, allstategear and transpartdistributors both list parts for these transmissions. I have purchased the 21mm input shaft from allstategear, and have no issues with it. So far it seems like a quality part. I have also ordered rebuilt kits from both sites in the past, same experience. If you can figure out which part you need and they have it in stock, I wouldn't hesitate to order from either