Steven P. McNicoll wrote:

This technology demonstration was only going to produce one winner, which
would have become the F-24 program without the intervention of the
previously mentioned DoD official. The first few produced would have been
prototypes and designated YF-24.

I know. My point was that the demonstrators could have used XF designations
(with numbers 24 and 25) without violating the rules and definitions of the
designation system. There is no rule saying that XF or YF can't be
allocated to competing designs of which only one (if any) will ever be put
in production.

The speculation is rather academic anyway, because the was a valid reason
why the JSF demonstrators used (a) X-designations and (b) non-sequential
numbers. From that point, the only logical designation for the forthcoming
prototypes was of course YF-24. BTW, so far only the plain F-35A/B/C
designators have been officially allocated, so there's no "YF-35"
designation yet.

Andreas