Why a 1939 Pearl Harbor could be even more vulnerable

Would a Japanese carrier attack on Pearl Harbor in 1939 be less devastating?

Probably not, because the base lacked much of the radar, air defence, and infrastructure built during the next two years.

A magically available six-carrier force could inflict similar or greater damage in 1939 despite using less capable aircraft. Pearl Harbor was already used for exercises, but it had not yet received the radar, air defences, and infrastructure added before December 1941. Those missing preparations matter more than a simple comparison of aircraft specifications. The answer is also explicitly counterfactual: Japan did not possess the historical six-carrier striking force in 1939, and an early attack would rewrite the wider war.