This is one time where knowledge about aviation development helps: a lot of experimental military aircraft are given an X designator to indicate that they *are* experimental. Once moved to production, the name would be shifted over to "F" for fighter or "B" for bomber or the like.

In Intruder, Rodney definitely says F-302, which makes me think that the name did get changed at some point, without bothering to tell the audience in the interim.