p6 prereg (4 Nov 10:10): joint fit, every ship gets two offsets (before/after 1863-07-01), sum-to-zero per period, robust LS, day-block bootstrap.
If 1863 steps are partner leakage (partners' drift absorbed by a fixed-partner fit):
 P1: among p4 steppers with cp in 1863, joint step |after-before| shrinks to < half of p4 step in >=6.
 P2: among p4 controls (drift False), <=2 have significant (>3se) joint step.
If steps are real instrument changes, P1 misses and steppers stay significant.
