That's the SLJ front derailleur from the 80's. I have a few of those in my stash and on my bikes. The black Delrin part with the Simplex logo is just a garnish cap over the gap between the parallelogram arms of the FD mechanism. No structural loads on it at all, so these pieces never really fail, just sometimes get lost when not snapped back in correctly.
Actually, what fails on those FDs are the aluminum clamp and body assembly that cracks at the hinge and at a slot molded into the aluminum, to house one of the pivot arms, where there are corners that are stress risers.