According to my notes due to the problems with the aircraft wing structure and the plane control at the high speed, especially during diving. beginning the February 1941 (A6M2 serial no. 3127) a servo tab (Flettner) was introduced. However, after the next crash, in May 1941, the thickness of the external plating of the wing surfaces was increased. In addition, longitudinal reinforcements were installed to increase torsional strength, and the ailerons returned to external mass ballance in place of the servo tab. The modernized planes were designated A6M2b Model 2 Ib. The external mass balance was added to the ailerons, also on previously produced aircraft, now designated A6M2a Model 2la. The planes had a maximum speed limit of 630 km/h and still at high speeds there were large forces acting on the rudders. The measures taken, as Mitsubishi representatives said, allowed the system of servo tabs to be left in place, which would solve the latter problem, but the Navy, after the disaster of Capt. Shimokawa ( A6M2 s/n. 3135 on 17th April 1941 ) ordered them removed, not being sure of the cause of the accident. As a result, they were only installed on approximately 80 A6M2 Model 21s built in February–May 1941.
In mid-September 1941, a new aileron began to be installed, which had all the counterweights inside the front section, eliminating the external mass ballance that caused unnecessary aerodynamic drag. The above change was introduced starting with A6M2 No. 327.
So there was the mix of Zeros with the external mass balance or without because their presence or absence depened on the period of time a particular plane was assmebled.