|1908-Vanguard 23J 1913/14|
Well as you can see, not a lot, this particular 1908 model has the better type of regulator but most 1908's have the star macro regulator seen here on the 1899.
The later model also has ruby & sapphire jeweling with diamond end stones to the balance and the 1899 is missing the diamonds but again only a minority of the 1908's had them.
|1899-Vanguard 23J 1904/05|
In fact there were some bigger changes to the 1908 during its extended life time than there are between the 1899 and 1908 models.
So the bottom line is that the model number change was as much a marketing ploy than anything else.
Some things never change.