"What is the difference betweeen the '1899' and '99' model designations? I see both used as seperate models?"
The model designation was changed right after World War I with the introduction of the .250 and .300 Savage cartridges.
IIRC, receiver steel formulation was changed, as was the heat treating process to allow for the chambering of the new high pressure cartridges.
I don't believe that there were any dimensional changes to the receivers themselves; if there were, they were minor, and certainly not to the degree that changes were made after World War II when the .308 family of cartridges was introduced.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.