The Parker '51' Aerometric filling system was introduced in 1948. In England the two '51' versions were designated 'Classic' and 'Custom'. This pen is the high end Custom version with a 12Ct. rolled Gold cap. This is an american pen made pen carrying a date code for 1948, it is the first of the aerometric Parker 51 pens.
The very easrly aerometrics had a polished aluminium filler sleeve with the instruction to press the bar six times, as in this case, this was quickly reduced to four. The clip is also longer in the earlier pens, although this did continue for a year or two.
The 14 Ct. Gold nib writes with a smooth fine to medium line. The 51 nibs were largely hand made so there is some variation between individual nibs of the same specification. The hooded nib arrangement precludes any flex but Parker managed to achieve a very pleasing 'soft' feel with their Gold nibs. This pen write easily with no pressure from the user, an important consideration at a time when pens were used for extended periods and fatigue was an issue.