Your knowledge about the IJN is always impressive. Based on messages on Japanese modeling boards when the 1/700 Fujimi Kaga was released, I thought the addition happened when Kaga was in for repairs in 1942 pre-Midway.
Do you have more info on the 3rd pair of supports? Were they added earlier than 1941? Was an official reason uncovered or was it a matter of finding a picture with the new supports in place?