The centerline in this short article means a light colored (probably white) line on upper surface of engine cowling of Zero.

This centerline can be observed on one Zero (type-32) taking off from the Zuikaku in one news file. Although some photos confirm existence of the centerline on Type-99 bombers, it was not known that Zeros had a same kind of centerline.

Aichi type99 bomber 1941 Ford Island. Offer: Mr. David Aiken

An interesting point is that one Zero (type-21), taking off before that type-32 Zero, did not have the centerline. It seems that some had the centerline and some did not have the centerline even when they belonged to same unit. It is said that a certain airplane was assigned to a certain pilot in an aircraft carrier so that the centerline might have been drawn depended on a pilot's preference.

After we noticed existence of the centerline, we searched the other examples. Eventually, we found that famous EI-111 and 653-111 seemed to have the centerline.
The centerline of 653-111 is seemingly like a line for synchronizing machine-gun firing but length and position is different from that of it.
In case of EI-111, it is not 100% sure if it really had the centerline or not. However, seeking possibility, we put the line on it.

Illustrates shows EI-111 having thick yellow lines on its wings and 653-111. Both airplanes belonged to units for aircraft carrier ships. The purpose of this centerline is not certain. One possibility is that it could be used as an indicator to help a pilot keeping his airplane straight to a carrier ship when he was approaching. If so, only beginners might have needed the centerline. EI-111 was for a leader but it is interesting to imagine that the leader drew the centerline on his airplane in order to show youngsters that it was no shame at all.

Reference
Shashinsyu Reisen
Incorporated company Nihon eiga shinsya "Collection of Japanese warplanes Volume on navy plane"