Component: L15H31_ATTN
Hook: blocks.15.attn.hook_z
Layer: 15
Head: 31
Role: Backup C-transfer head. In the full circuit this head is mostly redundant with L15H3, but when L15H3 is suppressed it becomes strongly load-bearing. Its functional route is last-token attention to the third operand C, providing an auxiliary C-transfer pathway.
Additional note: Secondary transfer head at Layer 15. It is not catastrophic alone but in the critical tail. Paper Table. 4 lists it in the secondary AF1 transfer set.