I'm thinking, if my memory is correct, that with a serial number in that range, 912XXX, it was probably made in the mid to late fifties. 1950 ended at about 566,000. The 308 Winchester cartridge came out in 1952 but I'm not sure what year it was first offered in the 99. I think 1953 is in the low 700,000 range. In 1960 Savage moved from Chicopee Falls, MA to Westfield, MA and the rifles before and after 1960 are thus marked....
There is a code on the lever boss:
Lever Boss Codes:
A = 1949
B = 1950
C = 1951
D = 1952
E = 1953
F = 1954
G = 1955
H = 1956
I = 1957
J = 1958
K = 1959
L = 1960
M = 1961
N = 1962
P = 1963
R = 1964
S = 1965
T = 1966
U = 1967
V = 1968
W = 1969
X = 1970
Besides the round counter, it should have a brass rotary mag follower, not aluminum. I'd like to give you more precise info but I don't have my Savage book or rifles handy at the moment. As far as what it is worth, 99's with serial numbers below 1,000,000 are considered the most desirable. As far as value is concerned I'd have to see a picture or at least a model letter but if it's in good to very good condition, somewhere in the $500-$700 (maybe more) range if I had to venture a guess sight unseen.