I always presumed this was a prototype done several years later, after the intro of the 99C. The model 88 was discontinued in 1973, so I wouldn't be surprised if this dated to shortly after that.
No date code on the front of the lever or above the lever on the frame?
“
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