Check this list on the Remington Society of America:
http://remingtonsociety.com/rsa/service
Not marked, Model 9
The Remington Model 12-C rifle was produced from 1910 to 1936. This model is a variation of the Remington Model 12, which was introduced in 1909. The Model 12-C specifically refers to the takedown version of the rifle, which allowed for easier transport and storage.
1920
Remington Model 12 barrels are threaded to the action. Barrels can be replaced with the proper equipment.
stocks are the same for any model 12 or 121, try e-gunparts.com or watch on gunbroker.com. There are people making them new as well for example http://macongunstocks.net/remington_model_12_or_121_2_piece_riflestock_96.html http://www.gunstocksinc.com/web_pages/Plate-pics/Remington-Stocks/Remington-Rifle-stocks.htm You can also try e-bay. Keyword Remington Model 12 and see what you might find.
Try Remington's web site. Go to remington.com and order one, they are free for most guns.
Model R-12
No such model is listed in the Blue Book or Standard Catalog.
Early rolling blocks and the years of the model 12 "pump" were not marked, need a good description to identify.
Are you sure it's an auto and not a pump action, it sounds like the predecessor to the Remington Model 10.
1919
Try e-gunparts.com There's a whole list of places on the Remington Society of America here: http://remingtonsociety.com/rsa/service