You must consider the general silhouette of the clothing along with other factors such as hair, price, sizing and envelope style and logo placement you can get pretty close to the issue or copyright date.Please be aware that even if a pattern is dated, it could have been in production for years and could even be a re-release of a previous number. For instance it is very difficult to tell a 1938 pattern from a 1941 pattern. The same thing applies for 1959-1961 and again in the 60s and 70's.
In mid 1967 the standard changed once again and a Size 12 became a 34 inch bust and remains so today.
So if you are trying to date something near the year of size change, the size can be a great clue.
The resulting chart is a work in progress, which I will add to as more information becomes available.
There are still a lot of blanks, but, even in a very sketchy state, it can be used to date patterns within a year or so.
So I know this is a sticky topic because unless you've actually seen the year printed on the pattern you might not trust what others tell you.