From j-aircraft.com….
Both Mitsubishi and Nakajima painted stripes on the outer side of the center landing gear strut cover. As weight was added to an airplane the compression of the oleos caused these markings to be progressively covered up. Mitsubishi painted three of these stripes, red, yellow and blue from top to bottom, while Nakajima only applied two of them, a red stripe painted on top of a blue stripe of double the width