The Forgeworld website suggests this theory. There are two ancient land raider variants done up in RS paint jobs. They each have a roman numeral II on them.
These are 10,000 year old tanks, the oldest in any legions' inventory...
And just for comparison, there is also an Imperial Fists tank with VII on it (they are the seventh legion) AND and Raven Guard tank with a XIX on it (the 19th legion).
So according to the Forgeworld painter anyway, when he puts roman numerals on a tank they refer to the legion's founding number.

So... Cool?
But where's the primarch?
And can anyone remind me of what the canon is for the missing 2nd legion? I don't even remember.