From around 1934 and into the 1950s, Parker pens (and pencils, though less consistently) were marked on the barrel to indicate their date of production, with nibs marked similarly. At first the code consisted of two digits: the first indicated the quarter; the second, the year. In the later '30s the code was changed to a single-digit form, with the year number surrounded by up to three dots. Three dots indicated the first quarter, and in each subsequent quarter, a dot would be ground off the die, so by the last quarter no dots remained. From 1950, the date was denoted by another two-digit system: "50" was the mark for 1950, and "51" for 1951. The system was dropped not long thereafter (the earliest US date code we have seen is "34"; the latest, "55"). Canadian-made single-jewel Vacumatics were produced into the 1950s, bearing a single-digit date code in a much larger font than that used for US production. A similar date code may also be found on other Canadian-made Parkers of the same era.
One will run across anomalies, where pens bear date codes that seem much too late. These seem to be the result of Parker clearing out stocks of old parts, assembling them into pens years after they were originally produced. Note that one will NOT run across pens with date codes that seem too early! The anomalously late date codes also tend to cluster around certain years and certain models, further indicating that they were not applied in error, or arbitrarily.
Parker once again applied date codes to its products starting in the later 1970s. This code uses one of the ten letters in "QUALITYPEN" to stand for the respective year digit between 0 through 9, followed by another letter in the sequence "E, C, L, I" to designate the quarter. "QC" would thus stand for the second quarter of 1980. Another method of designating the quarter was used from 1987 on, in which the year letter was preceded by three vertical bars in the first quarter, two in the second, one in the third, and none in the fourth.
NOTE: Many 51s from 1946-47 bear a "T" prefix (the examples we have seen run from the 2nd quarter of 1946 to the 2nd quarter of 1947, with the T6 date code being the most common). A commonly-repeated explanation is that the "T" indicates production at Parker's Canadian factory, in Toronto. This is certainly mistaken, for a number of reasons -- not least being that these pens are also prominently marked "MADE IN U.S.A."
So what does the "T" signify? Parker's Toronto factory was set up to get around the high tariff barriers between the United States and the British Empire and Commonwealth. Most Parkers marked "MADE IN CANADA" were destined for export, not for the Canadian market. "T"-marked 51s are found in the USA -- not in Canada or British territories -- nor do they exhibit any of the subtle differences that distinguish Canadian 51s from their US-made equivalents. At the time the "T" code was in use, the postwar surge in demand for 51s was outstripping Parker's production capacity. That demand was strongest in the USA, where there had been wartime shortages, but also full wartime employment at high rates of pay. The British postwar economy was much weaker, with no corresponding explosion of consumer spending, with rationing on basic goods remaining in place until the early 1950s.
Given this background, I speculated that the "T" code denoted the use of parts made in Toronto (either completely or partially) in US-assembled pens. Import duty from Canada to the USA would have been significantly lower for parts than for finished goods, and presumably only a limited number of parts would have come from Canada -- otherwise the pens would not have qualified as US-made. This would have been an affordable way for Parker to stretch its resources to meet immediate demand until it could expand its US production capacity.
More recently, however, a leading 51 expert came to an entirely different conclusion. This will be published shortly, and I find his case entirely convincing and strongly supported by all available evidence. I will not give away his answer here, but I will say that we can now be certain that the "T" has nothing whatsoever to do with Toronto.