The design of the Parker 51 was inextricably linked with the development of a new fast drying ink that Parker had developed in the 1930s.
The ink was, indeed, fast drying, and did away with the need for blotting paper but it was also strongly alkaline and contained chemicals that could damage the plastics used for the manufacture of most pens at the time.
Where most companies would have rejected the ink on those grounds, Parker saw it as a bonus in that they could make a pen from an ink resistant material and market it as the only pen that could safely use fast drying ink. The material was 'Lucite', later known in the U.K. as 'Perspex' and the pen was the Parker 51.
The pen was about ten years in development, but when it was finally let out into the wild it was a sensation. Not only could the pen use the new ink but it could deliver it to the paper smoothly and efficiently with no leakage, drying up, or starting problems.
The small gold nib, tubular in design, was enclosed in a 'hood', or 'shell' which kept the nib moist, protected it, and prevented it from drying and clogging.
In addition, a device known as an ink collector, or governor was invented by Marlin Baker. The patent extract, from David Shepherd's excellent Parker 51 book, explains the principle:
The collector, originally machined from Lucite, but later injection moulded, consists of circular 'fins' which trap the ink in the hood to essentially provide a second ink reservoir to feed ink directly to the nib, it is extremely efficient and accounts, in part, for the supreme writing ability of the Parker 51.
a rough calculation shows that about 0.25 ml. of ink is stored in the slots between the fins, plus a little more trapped between the collector and the hood. this may not seem much, but it is the equivalent of four drops, easily enough to do the job.
If a 51 is left unused for some time, however, the ink will dry and the fins become clogged thus preventing it from working efficiently. This can often be flushed out but sometimes it is necessary to dismantle the front end' of the pen and give it a proper clean, a good reason to look for a fully serviced pen if considering buying a 51.