The Parker “51”

In 1943, he and Parker signed an agreement with Sheaffer, forestalling pending litigation for patent infringement, under which Sheaffer, for a payment of $25,000 and royalties, was granted a license to use the collector concept that it had embodied in the conical feed used with the “TRIUMPH” point and was excused from penalties for the pens it had already sold that used the concept. For the first six years of its existence, it came in one size only; not until 1947 did Parker make a model of a different size — and then only a shorter version, called the Demi:

The metal cap exemplifies the pen’s advanced engineering. Embodying Parker’s proven and very attractive design for mounting a sturdy (and replaceable) clip, it slips smoothly and reliably over the shell to seat on the clutch ring (U.S. Patent No 2,278,907, by Marlin S. Baker) and then stays put in a way that was inconceivable to the makers of earlier hard-rubber slip caps.

Source: www.richardspens.com