What's the meaning of this marking on this old transformer?
S is for shield. This is a single turn of conductor placed between the primary and secondary designed to be connected to ground. The transformer has a significant stray capacitance between windings. Without the shield, any noise on the primary would be coupled to the secondary. With the shield, the noise goes to ground through the shield connection. The shield is a single turn of foil, insulated so that it does not form a shorted turn.

17-0-17 means a single 34v winding, tapped in the middle. This configuration is often used for symmetrical DC outputs, with the '0' connection going to output common.