But I don't see the diaphragm in the parts list, and it seems like it is concealed in the base? Yes or no
The tip of the launch boom rests on a on a large round "counterweight" portion of the swinging base. The rocket points at the counterweight. The diaphragm is inside the round portion.
It's a great mechanical assembly as far as performance goes. Mine original likely has millions on cycles on it, lol.
On the original blue and red one, the trigger mech got too sensitive so I added a spring over the trigger linkage that also urges it into action just a tad. But that little linkage part is what I consider the weakest link, except for maybe an aged diaphragm. I see some differences in old to new parts, but they seem compatible. It's a just a long teeter totter with the diaphragm at one end. The other end pushes the boom up.
The launch is started via a platic button under the rear of the boom (a black button on the blue/red car below is easy to see) The metal lever on the side lowers the boom and resets the boom lock in the lowered position. That position allows the launch spring to be set and a rachet mech locks it into position too. Just ahead of that lever is the trigger link. When raising, the linkage pulls out of its hole until it finally engages the body by its staggered hook end as it reaches full upright, and the final bit of movement the linkage pulls down on the trigger arm in the boom. My ramps on the teeth of the trigger lock are a bit rounded from use. The added spring keeps the pawl seated well. Just the drag on the link from sliding and rubbing in the hole would trigger it. A bump on the track or vibrations from a long run would too.
I also throw short cotter pins into the stake holes as stops. It is to prevent the assembly from twisting, creeping slowly out of being parallel with the car and transforming into a 6 inch wide beam of destruction. With extra weight to cause the swing, this guy likes destroying the layout any way he can.