First, the .243 Win cartridge was first introduced in 1955 by Winchester
in their Model 70 bolt action and Model 88 lever action, so it's got to
be more recent than that. (Assuming by .243 you do mean .243
Winchester...) Unless the barrel has been replaced...
You say you checked
savage99.com for the date, I found this page there:
http://www.savage99.com/savage99_dates.htm
Notice near the bottom they have a section called "Lever Boss Codes:".
Looks like Savage started embossing a letter on the lever to signify year
of manufacture from 1949 through 1970. See if you can find that.
If that doesn't help, try asking in rec.guns.