Here's my 1981 DK custom 3B A8 Trak-Ball, with the bottom view showing the metal mech assembly at upper left. The white envelope is the caddy for storing the 3B MC diskette. The plastic is transparent orange Lucite; the rear wall has collapsed and needs some re-gluing.  It's essentially the same circuitry as a CX22 in Trak-Ball mode that has a cable using its paddle lines (pins 5 & 9, I think) for the left & right fire buttons.  Pressing one of the outer buttons grounds the paddle line to Count 0, so it is read as an active low.  Anybody can build their own with the right cable and two more switches.  I think the 2600 keyboard has the right cable with all wires present.