In vertical month-year placement an alternate for the = dash was an underlined month numeral. Copeland & Sons, from about 1867 into the early 1900s, used an impressed vertical 3-digit date mark with the month letter over the last two digits of the year.
Because it has been in existence for more than 200 years, it is the china pattern with the longest continual production in history. It finds its roots in China, where throughout the 15th through 18th centuries, the Chinese potters were exporting their porcelain wares decorated with hand-painted cobalt designs under glaze.
In the 18th century, companies like the East India Company imported the blue and white Chinese porcelains into England.
This information is contained in many reference books.
However, the only definitive way to date a particular piece is to identify a pottery impress-date on the 'white ware' (the bisque-fired pottery item prior to decorating).
Unlike pharmaceutical ‘EXP.’ dates, there is no assurance regarding an impressed potting date and when an item was actually decorated with its date-attributable design.