For those of us concerned with when a pen was made, Parker is the sweetest of all makers, in that many of their pen actually have a date printed right on them. Not only the year, but which quarter of the year, will appear, giving a very clear sense of just how old the pen is.
This practice was evidently started as a means of helping quality control (“What was it exactly that we changed between April and June of last year that made the pens catch fire?”), and it is not as certain as one might hope. While modern Parkers that follow this practice mark only the barrel or cap of the pen, vintage pens generally have a code on the barrel and on the point. One generally takes the barrel as definitive of the age of the pen as a unit, since caps, barrels and blind caps were usually all made together, and swapping tends to show. Points, on the other hand, bear the brunt of the pen’s daily activity and are more likely to need replacement. In any event the point’s date code is frequently hidden inside the section, and are much less useful for dating the pen.
This does not mean that a point whose date code does not agree with the barrel is necessarily a replacement. Points and bodies were made in separate production lines, not meeting until final assembly, and the lines did generally run to the same schedule. The result is “mismatched” codes, about which I urge you to not be too concerned about.
An other thing to not get too concerned about is finding a pen has lost its code. They tend to be small and not quite as deeply impressed as the rest of the barrel markings, and decades of wear can render them obscure or efface them entirely. Modern pens are much more likely to present their codes, but their codes are less easily understood.
Parker began applying date codes in 1934. From then until 1938, the codes took the form of a pair of digits; the first indicating which quarter and the second the year. Thus, a pen with a 46 code is from October through December of 1936, and pens made in the third quarter are very obvious about their year.
In 1938, the codes for the quarters were changed to simple dots; the accepted reason for this is that it saved in the making of the stamps; rather than having a new code-stamp each quarter, a dot was merely ground off the one for the year. This means that the codes now had a single digit, flanked and supported by dots at the start of the year, dropping the one beneath in the second quarter, the one behind in the third, and standing alone in the fourth. I understand that this was not brought in until the second quarter of 1938, so the codes for that year should go from 18 to .8. which could rather confuse the unwary.
This single digit code persisted until the end of the 1940s, when a second digit appeared. It was decided that rather than run the risk of confusion with earlier models (apart from those made in 1938 and 1939) to add a second digit to the year, so a pen made in the third quarter of 1950 would bear a 50. code. This scheme was only pursued until about 1955 in the US, and was dropped elsewhere by about 1960. Parker “51″s made in Canada and England in the 1950s are exceptions to this pattern, as they stuck with the earlier single-digit pattern.
The hiatus in date codes ended in 1979, with an entirely new plan. The core of the code takes the letters QUALITYPEN and assigns them digits from 0 to 9 to represent the year. Initially, the quarters code was based on the a very square, eye-chart style version of the letter E, with elements removed for each quarter, producing what can be described as E, C, L and I, but the C is rather stylized, each following the year letter. These quarter marks were changed out for a simple Roman numeral count-down from III to nothing at all, ahead of the year marker, in 1988; this was either to mark the company’s centennial or to avoid a whole quarter of pens date-coded EE. Both methods allowed for the same subtractive changes to the dies responsible for the impressions.
Since 1988, there has only been one substantial change in the code, in that the quarter indicators were shifted back to after the year indication, with a dot separating them. This scheme, as of late 2011, appears to be settled upon for the current decade, so some more attention to finish and trim will be necessary to identify pens like the Duofold which span decades.