The CSX designation means it's part of the "continuation" of the original serial numbers. I don't recall the year, and I've forgotten most of the details (so feel free to call me out on something I've gotten wrong here), but apparently Carroll Shelby "found" (yeah, right!) a bunch of "original" frames that were numbered, and then forgotten about in storage somewhere. (How the hell does this happen?) So, despite them being assembled in the 90's (?) or so, Shelby got approval to "continue" the production of them, but built to buyer's Spec (modern drivertrains, anyone?), and they were even able to sell and Title them as '66's (or so).
I grew-up with Fords, and have a ton of respect for Carroll Shelby and what he accomplished Back in The Day, but this "continuation Cobra series" thing is pure B.S., as far as I'm concerned.
Just my opinion, of course.