Shadownone,
Actually the receivers on low number 1903's were very hard. The problem was that during the heat treating process of low number receivers no pyrometers were used to determine the heat treat temps, but rather heat treat temp was visual. The old pro's thought that there could tell by the color of the metal if the receivers were at the proper temp. This caused some receivers to be heated to hot which resulted in brittle, burnt, metal. As a result low number 1903 rifles were deemed a saftey risk to the shooter. Attempts were made to reheat treat the receivers to make them serviceable again without success. Unfortunately destructive testing is the only way to determine which of the low numbered receivers has burnt steel. Many shooters feel that these rifles are safe to shoot. Many shooters feel that these rifles are not safe to shoot. This subject has sparked many a heated debate among 1903 shooters. If you get a chance, pick up a copy of Hatchers Notebook. He explaines everything you need to know about the heat treating process as well as many other interesting things.
An "S" stock is a straight stock. As stated above, a "c" stock is a pistol grip stock. There is also a Scant Grip stock, which was a replacement stock with a less pronounce pistol grip. HTH.
Regards
Ox