The rear axle had two A-4205 Differential Gear Cases in 1928. I am not sure when the A-4206 Differential Gear Case went into production, but my guess is it came about with the change to the single disk clutch. There is no service bulletin about the A-4206 Case, but it must of have been a good reason for its use. If it was my choice to make, I would get the later combination of the A-4205 & the A-4206.
Perhaps a Model "A" historian will chime in with the facts of when & why the A-4206 went into Model "A" production.