Removing the barrel and manually squeezing the sac, directly or with an overlying bar, is a simple concept. Unlike the Eagle version, which would have seen like a cheap way to go at any time in history, Parker and others found a way to make this idea seem more elegant in the execution. By adding a sac guard, and metal bars to help fully squeeze the sac, Parker (and others utilizing squeeze filling converters) succeeded in making the simplest of self filling mechanisms seem like well thought out engineering.
