Above: A 51 Aerometric Mk1 from around 1950 showing the long clip which was in use from 1948 to around 1951 This aerometric filling system was used in virtually all 51s until 1978 when production in the UK ceased. Small changes and improvements to the 51 continued almost annually from 1950 on with the major redesigns in the late 1960s with a streamlined Mark 2 followed by the final Mark 3 version in around 1975. A wide range of 51 variations were produced in both the UK and USA with slightly different timelines, and it is not unusual to find different colours, cap styles and imprints in US and UK production

The filling system was re-designed in 1948, with the introduction of the Aerometric filling system. This filling system operated by pressing a pressure bar on "Pli-Glass" PVC sac. [2] The pen and the ink were both named "51" to mark 1939, the company's 51st anniversary, during which development for the pen was completed (U.S. design patent No. 116,097, U.S. Patent 2,223,541 filed). By giving the pen a number instead of a name, Parker avoided the problem of translating a name into other languages.

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Lauren Hubbard. "Queen Elizabeth Has Used the Same Brand of Pens for Almost 60 Years". Town & Country Magazine . Retrieved 10 Jul 2021.

In 1947, Parker’s factory in England also started producing 51s for the British and European markets and, with the Vacumatic filling system being seen as overly complicated, Parker worked towards a new type of filler which was longer lasting, more reliable and simpler to use. In 1949, the ‘New Aero-metric Parker 51’ was launched in the USA and described as ‘a clear case of improving on perfection’. Instead of the button operated rubber diaphragm filler, this new 51 came with a PVC pli-glass reservoir with a 30 year life expectancy - an astonishingly long life given that most pens then required a service every 5 years or so. In the event, even this projection proved to be a little pessimistic as 51s continue arriving here every month for their first service and sac replacement in 70 years! English production of the Aerometric commenced at Newhaven in 1950.

While the same design of collector isn't often used in modern pens, it's very normal now for pens to have a lot of fins connected to the feed, holding ink close to the tip and ready for use, even in rollerball pens. The collector lives on, influencing the design of many pens all over the world. The"51"was innovative for the period. It had a number of new design features—in particular the hooded, tubular nib and multi-finned collector were designed to work in conjunction with the pen's proprietary, fast-drying "51" ink. This allowed the tubular nib to stay wet and lay down an even line with either "51" ink or conventional inks. The initial model used a Vacumatic filling system which operated by pressing a plunger to generate a vacuum, drawing ink into the pen. [1] Above: In 1947-48 Parker produced a demi-sized version of the 51 Vacumatic. All parts except for the barrel and cap were identical to the full sized 51. An uncapped Demi is about 11.2cms long whereas the full sized 51 is 12.8cms The Parker 51 Aerometric Shepherd, David; Shepherd, Mark (January 2004). Parker "51" (1sted.). United Kingdom: Surrenden Pens Ltd. pp.102–104. ISBN 0-9546875-1-5.