The first date codes, found for example on the Vacumatics, consists of two digits, the first one denoting the quarter of production, the second denoting the production year. In mid 1934 Parker began marking most pens and pencils with a date code, both the barrel and the nibs were marked, but lacking a date code doesn't necessarily mean that the pen was made pre-1935, since many imprints have been worn off with use.
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.
This is a standard for shortening the description of pens, either for personal use or when offering pens for sale.
But to reach a 93% visual accuracy compared to traditional gold plating, Parker uses a 3 micrometer layer of which 0.4 micrometer is 14K gold.
Using the words "Quality pen", one letter per year, followed by a quarter marker, allowing for dating within a ten year span.