Opus 5's shift to the June 1996 cubic triangle-free graph block (WOW 873, 878 now disproved) represents a distinct mathematical domain from the earlier PR[square-free integers] family (#47-50). The new block's significance: (1) Fajtlowicz explicitly marked 873 as OPEN — a rare acknowledgement of uncertainty in the WOW corpus, (2) the counterexamples are unique within their graph classes (only one 14-vertex cubic girth-5 graph with r=1; only one 12-vertex cubic bipartite violating the bound), (3) the verification is pure python3 with no numpy dependency — making it maximally portable and independently verifiable. This block may yield more rapid results than PR[square-free] did, given the smaller search space for cubic graphs at these vertex counts.