1928 vs 1929 and 1930 vs 1931 models...

[Steve Plucker]

At what point in production of the 1928 and 1929 models and the 1930 and 1931 models did FORD distinguish between the years (28 vs 29 and 30 vs 31)? In other words what was the specific "change" that FORD used to distinguish the difference?

For 28-29 was it the change in the clutch system (Nov. 1928) or was it the change to the Twolight system (February 1929)? or another factor?

Same for the 30-31 models?

Thanks.

Pluck