One thing new collectors soon learn about Parker Duofolds -- Senior and Junior sizes, in particular -- is that a large number are now found with later nibs. Whereas the original nibs from the 1920s and 1930s bear a PARKER/DUOFOLD imprint, these replacements are marked with an arrow similar to that used on the nibs of Vacumatics. Though these are sometimes called Vacumatic nibs, those found on Senior-size Duofolds are not: they are shaped completely differently from Vacumatic nibs, and if you want to get really into details, the arrow has a different number of feathers. In fact, these nibs were specifically made as replacements. If you pull one out of its section, you'll typically see two stamps, "R" denoting "replacement", and a star indicating coverage under the original Duofold guarantee (see the 1946 and 1947 Parker Service Manuals, pp. 44-5 and pp. 57-8 respectively). When you find an older Duofold with one of these factory replacement nibs, it will inevitably be mounted with a replacement comb feed, which Parker considered an improvement over and upgrade from the original "spearhead" feed.
I have not made a systematic study of these nibs as yet. Going through examples on hand, I found none earlier than 1941. Quite a few bear date codes for 1946 and 1951, suggesting significantly larger production runs in those years. This may have been due to a surge of postwar repair demand, but could also reflect Parker's practice of periodically clearing out stocks of older spare parts by assembling them into complete pens.
ADDENDUM: Arrow nibs found in 1920s and 1930s Duofold Juniors do seem to be standard Vac nibs with added stamps to the heel. I'll follow up on this to confirm, and will update in a few days.
Early examples of replacement Senior nibs, date codes for first quarter and last quarter 1941. Production clearly began before the USA's entrance into WW2.
These nibs were produced throughout the war years. These examples are dated 1944 and 1945.
Postwar nibs are more common, however: these are from 1946 and 1948. Parker clearly took customer service seriously, continuing to provide warranty support for their former flagship pens decades after they were originally sold.