Doesn't explain 320. Also, another idea, seeing as we don't know how long the Spartan III's were being prepared for and such- although their designations are typically alpha-numeric, I believe the letter may just refer to their company (alpha/bravo/charlie, A/B/C), in which case if only one company was present, they would probably forgo the letter when using a call sign. No guarantee's, and I read Onyx so long ago I could be completely wrong about the company thing. And the timeline could be wrong, but I really don't know.
(My two cents)