The Parker Duofold aerometric pen was added to the U.K. range in 1951, although the ballpoint was a later addition, and went on to become a best seller. It was available as a set, most often a fountain pen with a pencil, but occasionally, and later, with a ballpoint.
The ballpoint was also sold as a 'stand alone' pen, but, judging from the relative scarcity, it probably did not match the fountain pen in sales.
This is a fine example in black with gold filled trim, it shows light usage marks and has been fitted with a new refill. It has a cap activated mechanism, operated by pressing the cap to extend and retract the writing point.