Rory, They did add 22HP, 303 Sav., 30-30 Win. and 300 Savage to the former Model 1899 250-3000 and called it the Model 99G.
The newest rifle I have with the 1899 stamp is a 99H in 303 made in 1925. I also have a 99C and a 99D both in 300 with 24" rifle weight barrels. Both were made in 1923 and the receiver ring is stamped Model 1899. These models were dropped with the introduction of the medium weight barrel about 1926. David
Oh, there were cartridge changes for sure. From what I'm seeing, the 300 Savage introduction seems to coincide exactly with the introduction of the 99 models.
But to Savage, I doubt that adding model letters to their rifles in late 1921 and adding a new cartridge to the lineup was any more of a change than adding the featherweight in 1905, or the Leader in 1904, or the 30-30 in 1900, or <etc>. It was not a ground breaking structural change.. in 1921, it was probably mostly marketing.
Sales guys: "Hey, it's getting hard to sell guns to folks that think this is all stuff from the last century. Can we change the name on our marketing stuff to help show our awesome new cartridge isn't a fuddy duddy black powder round?".
Production: "What do you want us to change?"
Sales guys: "Nothing, just let us call it the '99 and let's give letters to each type rather than call it stuff like the 22 inch saddle gun."
Production: "Fine. We could care less.."