In fact, Parker did set up a second production line to cope with postwar demand.
The Duofolds had their world premier in early 1921.
Marketing the red Duofolds was something of a risky venture since Parker previously, in the early 1900's, had tried marketing a red pen, the Red Giant, wich was disastrous.
There are some 1930s Parker date codes that have both two digits and dots.
Definitive documentation is still lacking, but it would appear that the dots in this context denoted the month of production within the quarter denoted by the initial digit.
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.
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!
For further information on the "T" codes, see Tim Laros and David Nishimura, "'T' is for Tractor: Parker 51 'T' Date Codes, Revisited" in the Pennant, Fall 2016.