Proposition: Additional copies screen the laboratory question

Add one perfect copy of each record. On the original laboratory interface $\{a,b\}$, all branch cross terms are screened, and Wigner's original effect has coherent probability $1/4$. On all four qubits, the effect projecting onto

has coherent probability $1/12$ at the symmetric point.

Proof. Every differing class has a copy outside the laboratory interface. The reduced state is therefore its record-diagonal mixture. Each supported basis branch has overlap squared $1/4$ with $M$.

The four-qubit effect has overlaps $1/2,-1/2,1/2$ with the three encoded branches, reproducing the coherent amplitude $1/(2\sqrt{3})$.