However, without installation instructions, especially for first time users like myself, it's hard to know if the additional files/folders within the folder titled "FSX Specific" should be exchanged with the default files/folders in the download, as they appear to be duplicates.
Originally, when the v18.3 first hit, they included everything for both sims, so they only had to make 1 download file for both sims. FSX, and P3D 64 bit. Now, due to the confusion, especially from the first time users like yourself, they split them off. Why Jimi doesn't include a Readme, or installation instructions, I don't know. It may be because they are releasing small updates, not enough to designate a whole new version, like v18.4 or whatever, too quickly, and Jimi couldn't keep up. I'm not certain, though.
I will speak with Jimi, and see if we can't get the installation instructions written up and included in a Readme for both download files.
During the summer, Jimi, Orion, and many other on the Team, are very busy. They are both still active duty military. This may contribute to their rush to release, along with the beta testers, and just general users crying out for various changes and updates. I wager they just ran so far behind, they gave up on the Readme file, or at least the auto-install file.
As I say, I will speak with Jimi, and see if we can get the Readme back on track.
Pat, thank you for taking the time to reply to our installation questions. I did install it exactly as you say and all the basics seem to work OK in the limited amount of time I've spent with it.
I am glad it's working for you so well

I am happy I could help out a little. I love this plane, if you couldn't tell, and tend to "advertise" it at every opportunity. Up till I started doing that, this plane was almost exclusively used by the FSX Blue Angels for their demo team flying. They have a lot of communication channels internally, so things like install instructions in a written form really weren't needed. With me shouting from the roof-tops, so to speak, they're getting a lot more "strangers", so a better Readme would be a benefit. Many first time installers aren't members of their forums, so that path of information dispersal, used up till recently, is no longer viable.
Do these specific files contain optional additional or more advanced capabilities or effects, or are they just left overs from previous versions, etc?
Mostly leftovers. No longer needed now that there's 2 separate downloads, one for each sim type. The two "Specific Files" folders can pretty much be ignored.
The only folder you need to worry about any more is the Extras folder. It has the effects that need to be placed in the sims \Effects folder, including their associated textures.
It also includes documentation on the aircraft, what it does, and how it does it. Very good information to read up on. Another excellent source of information on the plane is the real-world Hornet's NATOPS manual. I strongly suggest you download it in PDF format. That way, you have the information at your fingertips. You can download it here:
https://publicintelligence.net/u-s-navy-f-18-natops-flight-manuals/ .
That site has an immense hoard of various military manuals, both aircraft NATOPS of various sorts, as well as manuals published by the various services on a huge variety of subjects. Not just US Navy, but all services. Not just US services, but world wide. Great site.
I suggest taking a moment to browse around it and see what all they have you might be able to use
Enjoy it! It's great for FCLP, with scenery available from Paddles as part of his vLSO effort, as well as carrier ops. Bombing missions, A-A interdictions, the whole nine yards. It's even TacPac enabled.
Pat☺