Nowadays many people remember Simplex as a manufacturer of rather nasty, cheapo, plastic derailleurs that were rather imprecise and chattery. I know I often think of them that way. We forget that Simplex also made rather refined and silky high-end racing derailleurs that shifted notably better than their revered Campagnolo contemporaries. At least if you used them on close ratio freewheels.
Part of the reason for this gallic smoothness (and notably non-gallic quietness) was a small black plastic flange at the pusher plate of the outer pulley cage plate. Here is the patent that describes it in detail. I rather liked it.