Component: L16H21_ATTN
Hook: blocks.16.attn.hook_z
Layer: 16
Head: 21
Role: Primary A-transfer head. At the final query position, this head selectively reads the first operand A and transfers A-dependent information into the last-token residual stream. Corrupt-A patching through this head almost completely redirects the model toward the A-corrupted answer.
Additional note: A-Operand transfer head at L=16. Paper Fig. 5 states that it attends from the last token to the first operand A; the model effectively cannot solve A+B+C without it.