Possibly anticipating the 1980 official start of date coding in France and the UK, this could have led to the presumption that the dating actually began in 1970, but proof of this has still to be found.
The quarter marker changed in 1988 from E, C, L, I to III, II, I or none (last quarter) according to the below table.
In 1950 a new system for the date coding was introduced where the two digits only indicated the year, not the quarter hence "50" means made in 1950, this system was used in the US until 1955 and in Canada a few years longer..
It has been assumed that Parker began marking some of their pens on the trim or cap bands to allow for dating in 1970.
This is a standard for shortening the description of pens, either for personal use or when offering pens for sale.