Adam,
Look in the parts catalog for serial number specific variations for your 317. PC 1698 lists axle and ring & pinion versions breaking at SN155047 in sections 50-4 and 50-8 respectively. Similarly in sections 50-5 and 80-2 there are changes in the sight tube and fender pan mount that occurred at SN155,000...and in section 80-6 the strap that holds on the medallion, M81685, was added at SN190,000 and above.
Hope this gives you an idea of a place to start, good luck in narrowing down the SN range of your particular tractor.
Here is the year to SN table from the Model Info page on this site to give a mapping of these breaks to the year model, essentially pre- 1981 for tractors below 155,001 and post 1982 for tractors above 190,000 so the above should provide some bracketing of the possibilities for your tractor.
Year Serial Number
1979 95,001 - 120,000
1980 120,001 - 155,000
1981 155,001 - 190,000
1982 190,001 - 222,000
1983 222,001 - 285,000
Chuck