Ink for fountain pens will always be a compromise, it shall dry fast at the paper, but not so fast that it dries in the pen nib. Manufacturer of ink has always tried to make the perfect ink and during the thirties Parker had succeed with a new ink that was very promising. The usual ink dries through evaporation to the air, Parker's new ink dried into the paper. There was only one problem with the new ink, it was very aggressive against the rubber bladder and could not be used in a conventional fountain pen. So together with the development of the new ink they also had to develop a pen that could handle it.
The result was the Parker 51.
The development of the pen was finished in 1939, which was Parkers 51 anniversary, that's the reason for the name. Furthermore the name 51 worked in all languages and was easy to combine with the company name, without predominate over it.
The 51 was revolutionary at it was released at the market in 1941. The nib was hidden in the belly, it had a new type of collector and a metal, slip-on cap. At the beginning it was equipped with the same filling system as the Vacumatic, which was later replaced by the Aerometric filler.
The pen became a huge success and the high price also made it to an status symbol. It says that it was possible to buy only the cap to wear in the chest pocket, and giving the impression to own a Parker 51.
Some time between 1946-48 Parker stopped advertising the "Parker 51 Ink" and instead referred to the regular Quink.