If "originality" is a concern, avoid sanding the stock. Trying to preserve original markings and contours would be top on my list. Various solvents may work, but it all depends upon what finish was put on the stock. Try to find a solvent that cuts the finish. If all else fails, try carb cleaner or gum and varnish remover from the auto parts store with lots of rags and paper towels. That, along with a soft scrub brush, should get decent results. Then, a scrubbing with warm water and a soap (perhaps Murphy's, Simple Green, Dawn) and a thorough drying. It will likely look nasty until you oil it. For that I use BLO (boiled linseed oil), the first coat a 50/50% mixture of BLO and turpentine or mineral spirits. Put it on heavy, and wipe off excess after about ten minutes. Let it dry a few days and put on another several coats of BLO by rubbing it in by hand not leaving an excess that appears wet. Let dry several days between coats.
Some folks prefer tung oil. There are a lot of opinions on what is best, and it depends upon what you are starting with and what you envision in the end.
Special Note: Use the solvents in a very well ventilated space - preferably outdoors - and never near a source of ignition, and towels and rags should be laid out or burnt at the end of the days. BLO and dome other oils, etc. can spontaneously combust. (As a firefighter I have seen this happen more than once over the years with great loss.)