I'd say it's quite likely a 99RS. They do exist, they catalogued them for the entire 50's. Yours has all the signs of one. Be nice to get some good pictures of it when you get the time, the more we document and compare the more we learn.

And very interesting to find out they case colored the tang screws on yours without the rotor being case colored. Wasn't sure if they were only doing that at that same time or not. Another data point is always good.

“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