That sounds like a good bet. (Besides, if it had been a James Bond reference, you'd have people grousing that the movie used a DB5 to show off Aston-Martin's newest model, while in the novel the chapter title was "Thoughts in a DB III"
Interestingly, the designation next to the USS Enterprise appears to be VH-5 / VB-5 - so I'm not quite sure (if DB means Docking Bay) what this designation could mean...
(Vehicle Bay 5, Vehicle Hanger 5???)
The smaller constitutition class starship (the mini-connie) has what appears to be the designation: AF / AP, while the even smaller Excelsior starship has VB (?)
The plot thickens...
Hm. So maybe the second letter refers to type and the first letter refers to general class. Vehicle, Heavy (Cruiser) 5?
Meh. Now I'm just reaching.
I think you may be onto something! Rather than being obvious abrieviations, the letters may indeed stand for individual designations (as you've suggested), not unlike a vehicle registration.
With the powers of our communal Trek BBS intellects, this code will be cracked!