$500 is about the bottom price for a good condition non molested Savage 99. It goes up from there and the occasional deal is found below that.
Somewhere in the production they changed the cartridge rotor/follower from brass to steel. The ones in steel I have seen are color case hardened (like the lever) and sometimes have the colors removed on the edge that you see thru the hole in the left side of the frame. Just makes the stamped 'round count' number stand out better.
I don't know if they later made any followers in aluminum or not. I don't recall seeing any.
I have a 300Sav cal 99 here now with the steel rotor and the rifle dates from about 1954 IIRC. Could be a replaced rotor of course..
Everyone likes the brass one better but they both work just as well.
Not much goes wrong with them unless someone decides to 'improve' them. Touching up the trigger pull can lead to a dangerously light pull that lets off unexpectedly when the rifle is bumped or set down. The sear engagement isn't what it first appears to be and the wrong areas get worked over sometimes leading to the problem.
As long as there is a strong engagement w/ a normal let-off of 4lb +, should be OK.
Check the other normal things ,,feeding and ejection especially from a full mag. Check the safety for engagement. Check the butt stock for cracks especially around the top rear edges of the receiver.
Should be D&T'd from the factory for a top mount,,two on the recev'r ring,,two on the rear tang,,that's it IIRC. I don't think the 308's were D&T'd for the tang site but maybe early production was.
...It shouldn't look the 300Sav that I have here that has 9 extra holes in it to fill....