I would guess that the S501 designation is a shop fabrication mark that calls for a #5 bar, with a sequence number (01, 02, etc) for various lengths and bends. In 1967, I would also guess that the bars were Grade 40, but if you can get a piece of one (in a non-stress condition) and test it, you can verify the strength. Grade 60 bars, I think, were just coming into play in the late 60's early 70's.