Well, technically, as of today, pre August 1, 1964 guns are C&R eligible but I don't think many sellers will want to cut it too close, and it's not really possible from the information available, short of a factory letter, to nail the date exactly. The thumbpiece ("latch") change was in 1966, so that alone won't be enough.
The 50 years old is based on manufacturing date, not shipping date. Unfortunately, the former is not usually available, while the latter is. Certainly, anything with a SN below 300000 would be in very safe C&R territory.