Here's an exploded parts diagram from Robert Broderick's velo-pages.com Gallery (see the bottom part, the SLJ):



It doesn't seem likely to me that the "cup" piece B would be keyed; looks like it's just there to contain the wound spring.

My bet is that to install and tension the spring, you put your part A (3563L) into place on the back side of the derailer, put those on the edge of a table top or some other flat surface to hold them together, install the cup B (3561) and the spring (3885) from the outer side (through the holes in parts A + B), wind the spring up from the exterior and hold its free end into the slot in the edge of the casting (more easily seen in the bottom section of the exploded view) using a flat, thin implement, and then install the pivot bolt, keying it into part A. But I've never taken one apart yet.