Thanks to Rob (info. exchange page) for supplying this photograph of his rare developmental C-73599 tricolor crt.
RCA's 15GP22 electron gun and their developmental C-73599 tricolor crt electron gun are alike. Here you see convergence electrode G4 to the right. On the left, the isolation of pin 13 from the other pins can be seen. In addition, the internal lead that conducts the 10- to 12-kV convergence signal from pin 13 to the G4 (convergence) electrode is visible near the center of the picture, sloping downward slightly and terminating in a spot weld on G4.