Japanese planners put Hiryū's island to port so paired carriers could present their islands away from one another. The arrangement conflicted with the instinctive direction many right-handed pilots used when avoiding an obstacle during a troubled approach. It also left Sōryū's starboard exhaust route in place, forcing extra internal support and trunking for an island on the opposite side. A genuine mirror design would have moved the exhaust with the island instead of mirroring only what appeared above the flight deck.
Notable Point · 2:32:28-2:35:53
Hiryū mirrored the island but not the machinery
Hiryū's port island was paired with starboard exhaust trunking inherited from Sōryū, adding internal complication without creating a true mirror-image carrier.