The canes / dates as shown in Richard More's wonderful pages do not indicate a particular start or end date for the use of any of the canes. They simply show that there are a range of date canes for many of the years of production.
All we can say is that the canes included in a dated weight were made in or before the year of the date cane. There are many examples of Strathearn weights using canes from the years 1956 - 1964 when the company was named Vasart Ltd. And there are also plenty of examples of Strathearn weights using canes that may have been made during the even earlier period of 1946 - 1956 when the comany was first formed as Ysart Brothers Glass (tradename Vasart).
Not only that, some of the canes in Strathearn weights all the way through to 1980 could have been made in the 1930s!
However, there were certainly canes made in the Strathearn years only. But because there are no known catalogues of those (or earlier) canes, we can only determine a general style or perhaps colour and then check to see whether they been found in weights made before 1964. Not an easy task!
Edited to clarify the last paragraph.