A pretty good investment, eh? It was named the Parker “51” because Parker felt that they needed a name that wouldn’t be hard to pronounce in any country and also easy to remember. Also at this time Route 51 was built through Janesville and it is quite probable that this also was a reason for the the name. Furthermore It also came out of research in 1939, the 51’st anniversary for the Parker Pen Company.
More interesting info at: http://parkerpens.net/parker51.html