It was my thought that it could just be a pre-rendered model of the Longsword, but I also feel it is quite possibly a Skyhawk as well.  I don't believe any models or conceptual art has ever been released showing what most of the UNSC/Covenant arsenals look like.

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Finis Coronat Opus! - "The End Crowns the Work!"

"Seek freedom and you shall be enslaved by your desires. Seek discipline and there you shall find your liberty." - Bene Gesserit Precept, Chapterhouse: Dune