The Parker 75 'ancestry'
The 75 was launched in 1964 but it's roots can be traced back to the Parker VP which was introduced in 1962. The 'VP' derived from 'Very Personal', referring to the way that the pen could be set up for use by a particular person, it also had 15 choices of nib style.
The VP was a bold attempt by Parker to appeal to 'serious' fountain pen users and was moderately successful. Unfortunately one design element was disastrous and led to the pen's early demise. The filling system used a refillable cartridge that was similar to to a Parker ink converter but was intended to be removed from the pen for filling and then re-inserted. The end of the cartridge was a long, thin plastic tube with a hexagonal cross section. Inevitably, many users would twist the cartridge to aid removal and break the plastic nozzle, rendering the pen useless. It's odd that, after years of research and development, this wasn't addressed before production. By 1964 the Parker 45 cartridge/converter system was proving successful so the VP was scrapped and the system incorporated into the VP's successor, the Parker 75.
The 75 cut down on the number of nib options but retained the ability to personalise the pen in terms of nib set up. The triangular section allows the pen to be held in the same orientation each use and the nib can be rotated to give the optimum angle of contact for a particular 'hand'. The idea is particularly attractive to left handed users and, as a left hander myself, I can testify to its efficacy.