Have to admit that I missed the slot on the barrel. It is from the last year the 99H and 99T were catalogued, but it was the beginning of the year so I'm surprised they were swapping parts. Maybe Savage presumed 99H's in 300 Savage would sell better than they did, and so made up a bunch of barrels and had leftovers.

“The Savage 99 Pocket Reference”.
All models and variations of 1895’s, 1899’s and 99’s covered.
Also dates, checkering, engraving.. Find at www.savagelevers.com