In the model of how Tullio did early prototyping no doubt.
I would consider using a stainless steel set screw, Allen head or slot, maybe a drop of loctite blue if necessary.

the broken cast piece is no doubt zamak ( a zinc alloy ) same material that the upper Rally body was cast from… one would think with the preceding issues they would have tossed that material aside.
”fresh thinking, young minds, condemned to repeat mistakes”