I think that we can honestly discard the "is it a 2" discussion, as unless you look at the digital version (that is - look in the book, people) you can actually see the whole number, leaving no doubts open as to whether it is a neck or something else. The entire number 2 is visible. So, let's leave that behind...
There are a whole lot of other arguments against the "lost legion" hypothesis though, including it being a company banner for some other Legion, and the non-roman character of the number itself, while the legions are almost always presented with roman numerals. The most likely candidate seems to be an Alpha Legion company banner, and therefore it would be nice to know exactly when they adopted the hydra heraldry and even when they were founded as a Legion.
As for the "we will never see a lost legion in fluff so this can't be it" argument, that's circular reasoning
Still, it's a vain hope and a far fetched idea, but it would be kind of cool if it really is an easter egg.
Cheers,
BTB