Iceman refers to the Rapier I as "new" in WC1, and the Rapier II(?) is supposed to be F-44. I have a hard time imagining that the Hellcat V should be a design older than the Rapier.
Possible explanations include that the Hellcat I,II,III or IV could be F-42 and the Hellcat V has another designation. Or whatever.
Also I think somewhere it is hinted at that the Hellcat is supposed to be newer than the Thunderbolt and that one is F-66.
It has actually been explained before (though, admittedly, never in an official source - rather, the assumption is that if it looks just like the US designation system, then it probably works like that, too) that the Roman numerals after the ship name are not new versions of the same fighter, but rather new fighters bearing the same name as a previous fighter. So, the Thunderbolt VII is the seventh ship in the Confed designation system to be called the Thunderbolt. Each of the previous Thunderbolts, however, was a completely different ship. For a real world example, look at the F-35 Lightning II - it's a brand new plane, not even in service, but it's got a II, because the US already had a plane called the Lightning (back in WWII).
The specific version of a given fighter type is the letter listed after the designation number. So, the F-44G Rapier II is the seventh
revision of the second
Confed fighter called the Rapier.
The Rapier, incidentally, is something of a troublemaker in all this. The original WC1 Rapier was just the Rapier, not the Rapier I nor the Rapier II - though the WC1 manual did not have proper designations for any fighters anyway. WC2 introduced the Rapier II, and presumably the implication at the time was that the Roman numerals denote major revisions (similar to British, and the Kilrathi, who have the Dralthi Mk. whatever), while the letters perhaps denoted smaller revisions. At least, we can safely assume that was the idea - certainly, nobody had thought of the WC Movie back in 1991, so the "II" in the Rapier's name indicates that the WC1 version would have been thought of as the Rapier I. As you can guess, the Movie changed this, by introducing a far, far older Rapier. Strictly speaking, this is a big, glaring inconsistency in the WC Universe - but fortunately, because the WC1 manual did not include proper designations, were can just pretend that had it included proper designations, the Rapier would have been the F-44A Rapier II.
Ahem. Let's move along, shall we?
Getting to the real point - can the Hellcat really be older than the Rapier? Yes, of course it can - if we see Rapiers in 2668, why can't Hellcats seen in 2669 be contemporaries? But does it make sense? In 2673, in the WC4 novel, we read about Border Worlders flying Rapiers, while Confed flies Hellcats, and it is implied that Rapiers are dreadfully outdated. As a matter of fact, it does make sense, and we see similar things happen in reality all the time. In WWII, there were numerous cases where the development of a given fighter was discontinued because, for example, engine production limits made it impossible to continue producing new versions... or, on the other hand, there were cases where a given fighter was just pushed and pushed onwards even though newer designs outperformed it, because shutting down its production line (a necessity, if you want to switch the production line to a new design) would result in gaps in supply.
Also - this is well worth considering - there were many, many situations where a fighter "older" by designation was newer in production. Imagine, for example, that the F-42 and F-44 designs were both developed at the same time, but the F-42 ran into difficulties. Maybe its designer needed to help out with another project that was having difficulties, maybe something wasn't working here, or maybe something changed in the meantime to make this project a lower priority. Whatever happened - suppose the design was delayed by a few months. When they got back to working on it, technology had moved on - so now they needed to update the design, rework parts of it, and this generated further delays. Or, alternatively - suppose the F-44 was so amazing, so promising, that additional resources were poured into the project, and it was completed months or years ahead of schedule. In either case, by the time the F-42 would finally see the light of day, the F-44 would have already been in service for a couple of years... all of this, of course, is made up on the spot as an example of what could have happened. I'm not saying this is what happened, I'm not saying it's even a reasonable guess - I just want to illustrate the fact that numeric designations do not necessarily mean age.
After all the fans who came up with the F-86 many years ago put a lot of thought in it. I like that number and I think it fits well (and not only because the real world F-86 was a pretty badass airplane I liked a lot).
I don't remember for sure who it was - I think it was the Fleet Tactics website. The trouble is, their claim was that you can make these numbers out on that WC3 video. They never claimed that this was a number that made sense for whatever reasons - only that they thought they could make it out on a video that nobody else can decipher. Did they actually have other reasons why F-86 made sense? Maybe, but since they never said so, we can't really assume they did...