The 51 is one of the best fountain pens ever produced.  Parker incorporated a number of innovations on this model. The hooded nib allows the pen to use faster drying ink. This allows the pen to put down a dependable line of ink that is dry almost immediately. They used metal caps that slipped firmly on the pen making it easier to use. The first models had the dependable Vacumatic filler system that allows a large ink supply. The later pens use an aeromatic filling system that is both effective and durable.

Overall these pens are superbly engineered and very well constructed. They were very popular in there time and the pens that you find now demonstrate the quality and durability of these pens.