Component: L20_MLP
Hook: blocks.20.hook_mlp_out
Layer: 20
Role: L20 MLP writes a latent arithmetic feature that depends on all operands. Its last-token output is useful for the arithmetic task and depends on the operands, but it does not directly write the final answer or a clean intermediate such as A+B/B+C. Downstream layers appear to use this latent feature to form the final answer direction.
Additional note: L20 is the retained representative of the graded L17-L21 causal MLP block. Direct zero-ablation drops acc 1.0 to 0.737. Corrupt-operand patching gives balanced flip rates across A/B/C (0.253/0.242/0.273) with corrupt-answer logit gains +2.03/+2.20/+2.31, showing that the output carries operand-dependent task signal without isolating a specific arithmetic operation (i.e. it does not do any intermediate A+B or A+C or B+C type calculation). L20 is better described as a latent feature-builder.