OK, so I have both the 737 and 727 base packs installed in FSX steam version, the installation of both went normally, the aircraft are both present within the sim, and seem to look and operate normally. Here's where things get a little weird... There are no "CS-737" "CS-727" folders or sub-folders (or whatever they would be named) in FSX\SimObjects\airplanes, they simply aren't there, yet they are in the aircraft selection menu within fsx, along with all the installed liveries, and they load and open normally when selected.

The only captain sim folders I can see in FSX are under the main directory, ie.  FSX\Captain_Sim\, and under that folder there are 727 and 737 folders with 5 sub-folders in each, but none of these sub-folders are named like the conventional aircraft subfolders, ie. Model, Panel, Sound, Texture.

Like I said, things seem to work normally within the sim, but this setup makes it impossible to add 3rd party re-paints or perhaps changing engine sounds etc., as the necessary folders don't exist.

I've never seen anything like this, so if anyone has any ideas or suggestions i'd sure like to hear from you

The folders have to be there somewhere. I have both the 737, and 727, and all of the expansion models, installed in FSX-SE, and all of the CS_B727 and CS_B737 folders are in the D:\SteamLibrary\steamapps\common\FSX\SmObjects\Airplanes (PC), or D:\Steam\steamapps\common\FSX\SmObjects\Airplanes (laptop) folders.
Are you sure you are looking in the correct installation folder for FSX-SE?

Well, I figured it out, your post made me take a much closer look at my SimObects\airplanes folder tree.

They are indeed there, they were inserted right below a long list of very similar (almost identical) looking folders with the same lettering, ie. CLS_XXX etc., and I just missed them in my haste, and assuming they'd be named Captain Sim or something like that...

Anyway, thanks for getting me to slow down and really scrutinize the SimOjects\airplanes folder tree.. 8-)