This shows how the upper stage sits on the booster before launch

This is how the stages mate together. The other two are removed so you can see how the one stage sits.  The second stage's A10-3T motor is to be friction-fit into the upper stage rocket, and the nozzle end that sticks out a bit rests in the booster stage's plastic manifold.  Half-way up the rocket, a launch lug is attached to keep the top half with the wood launch rod.  Nothing really keeps it there except for gravity (and the extra G's during boost). I'm worried that the first puff of ejection from the booster motor will pop off these upper stages before they ignite, then they fall like a lawn-dart to their destruction.  Maybe I'll snug up one or two with scotch or masking tape before the first launch.