This article isolates an exact local cancellation mechanism in Lorentzian Regge calculus. The finite source convention is stated explicitly: squared-edge variables, a six-mode spatial perturba- tion map at τ = 7/10 with anchor T0 = 7/5, fixed Lorentzian angle branches, and an interior-hinge flat-linearized Regge response. For a hinge with six incident four-simplex sectors, let ασ be the sector-angle derivative covector on the mode space (XX,YY,ZZ,XY,XZ,YZ). If the sectors form a complete simply-transitive S3 orbit and the source is exactly equivariant, then 6 σ=1 ασ = 6ΠS3 ασ0. Thus cancellation in this branch is equivalent to absence of a trivial-isotype component; sectorwise stationarity remains a distinct mechanism. In the natural spatial permutation representation, the Reynolds projector has rank two and the test reduces to vanishing diagonal and off-diagonal coordinate sums. The theorem is realized by a rooted-Kuhn Boolean rank-1/rank-3 family and by a completed warped non-Boolean S3 star. A source-only census covers 115 authenticated occurrences, representing 74 distinct triangle tuples: 43 are exact zeros, split into 31 orbit cancellations and 12 stationary cases. Prospectively frozen deterministic tests cover one recurrent Boolean occurrence, 18 catalog-transfer occurrences, and one non-Boolean occurrence, with maximum float64 decision residual 3.63×10−15. A separate 90-digit reconstruction gives 2.51×10−91. The result is conditional and finite-scope, not a global classification, arbitrary-mesh invariance, continuum limit, or nonzero- amplitude law.