The pen that Parker referred to as the '51 Standard' is one of the least well known of the 51 variants and is a very rare pen.
This example is in American Burgundy:
From the picture the pen is indistinguishable from a Parker 51 Classic (the American pen was actually called a De Luxe). It has a Lustralloy cap and a 14CT. Gold nib, suppled by a 'normal' Parker 51 collector and feed arrangement. The material, finish, and build quality are the same, as is the cap and end stud. So, what makes it a Standard and not a Classic, or De Luxe?
The answer is easy to see when the barrel is removed:
The filler arrangement is the 'U bar' Aeromeric, as found on the U.K. Duofolds and the Parker 51 Special. The history of the pen now becomes clear, the filler sleeve is marked 'Parker 51 Special', very confusing as the rest of the pen is 'pure' Parker 51.
The consensus among enthusiasts is that, when Parker stopped making the 51 Special, they were left with a stock of Special filler units. Ever parsimonious, Parker did not want to waste this stock so chose to use it up on new Parker 51s.