On the GS RD. If there is a cable adjustment nut, 1953-1959. No adjustment nut, 1960-1963. Open C on the back, 1953-1957. Closed C, 1958-1963.
The lock nuts on the GS hubs will have a date stamp.
+1
There may also be a date stamp on the saddle. It would be on the cantle plate, so underneath the back, probably on the right side. Two digits plus a letter. The digits refer to the year, and the earliest I have seen was 59. I don't know what the letters mean, and I've only ever seen A, B, and C.
That style saddle was introduced in the mid fifties.