The Parker 51 is an excellent example of the successful integration of industrial design and style. The only major functional difference between various iterations is the filing system. The earlier models were vacumatics (identified by having a blind cap) while the later ones are aerometric. Aside from that, the next difference would be in the choice of nib (both in width and material) followed by the barrel colour and material. Of course, the cap also has a choice of either being made of various grades of gold or lustralloy. There are also many customized 51's available. Whichever model is used there is a very good probability that it will be a smooth and reliable writer.
Grand Vision Pens delivered to me a wonderful teal blue barrel colour model with a lustralloy cap. Pen is exactly as described and came well-packaged. Service and quality of merchandise from this vendor is excellent.