Till recently... As I saw another one, but not in mint condition, without serial number.
I have the same intuition than yours, on no serial number=no issue, and probably, replacement case.
Now, the thing is to know WHO engraved the serial number: Blancpain, at the factory, Tornek at his shop, or the US Navy?
Which may have an impact on the answer to know if they were issued or not, for cases which obviosuly had a life, seeing its condition.
Now, like you, I prefer a military Watch in used condition, rather than in mint condition.
Best, and thanks for sharing your views, Billy.
Nicolas