Don't use the word transitional... always confuses things.
That serial number should put it into mid 1925, and according to Murray's it is an 1899A. The 1899A was made up through 1927.
Now... why all of the other 1899 models transitioned into the 99 series around 1921 but the 1899A stayed rather than become the 99A is something I've never seen addressed. But that's what it is. Nice gun.
