I'm afraid there's an awful lot that I disagree with in this thread.
Although I do not slavishly follow original repair manuals, in this instance Parker's original procedures regarding fitting the shell to the nib using heat are still the standard, and much preferable to removal of material from inside the shell. This fitting can involve either tightening the fit between the shell and the nib, or adding additional clearance so that the shell does not pinch the nib's slit closed. This is as much a part of setting up a Parker 51 as is heat-setting a hard rubber feed on a conventional pen.
When one comes across a 51 with a feed that stands proud from the nib, it is not because of the effects of gravity over a period of decades (which is nonsensical to begin with, given that pens are rotated every which way, not consistently stored horizontally and feed-down), but more probably because the feed ended up heated to the point of softness when the shell was being fitted to the nib, and when pressure was applied from above to the shell and nib, the nib temporarily deflected downwards, carrying the feed with it. Once the pressure was released, the nib then sprang back, but if the feed had cooled by then, it would have taken a set.