The Parker Duofold U.K. aerometric filling pens were made from 1951 into the early 1970s. They now represent excellent value, particularly when compared to a new pen as they all have solid Gold nibs and are renowned as good writers. A new fountain pen with a Gold nib is considerably more expensive then a Duofold, and probably not as good.
In terms of choice of trim levels and colour, however, they are sometimes seen as a bit limited. There are only four colours, red, blue, black, and green, and all have the same Gold filled clip and cap band.
Discounting the Parker Lady, as there were a number of iterations, the Duofold came in four sizes: Maxima, Senior, Standard, Demi, Junior, and Slimfold.
For the prospective Duofold buyer a visual comparison of the relative sizes of the pens may be of interest. I don't have a Maxima at the moment so it's not included, but it is bigger than all the others, as it's name suggests.
Capped, in descending order of size: Senior, Junior, Standard, Demi, Slimfold:
Posted, in descending order of size: Senior, Junior, Slimfold, Standard, Demi, note that the Slimfold has jumped two places:
