Dating parker 51
A short model, known as the Demi, was introduced shortly before the Vacumatic filler was phased out.
Aerometric Demis are slightly longer than Vac-filler Demis, and more slender.
Demonstrators in clear Lucite were made for dealer use, but not sold to the general public.
The pen was not named after the P-51 Mustang fighter plane; but Parker took advantage of the coincidence by comparing the pen and the plane in its advertising.
The 51 Special was a later addition to the line, an economy version without a gold nib and with a simplified Aerometric filler mechanism.
Specials also usually have caps that are polished bright rather than frosted, and a cap jewel that is black, not pearlescent (note, however, that late-production Specials from the 1960s resembled normal 51s, with frosty caps, pearl jewels, and gold nibs).
Perhaps no other fountain pen has been so unanimously acclaimed as a classic.
Nor is it merely a triumph of styling -- though it has a place in the Museum of Modern Art's design collection, and Lazlo Moholy-Nagy found it worthy of praise -- for it is also one of the most robust and practical writing instruments ever made.