Why do some A’s have a Zenith carb and others have a Tillotson carb? Is it by model or year?
All the OEM Model A's were either Holley or Zenith carbs (look the same, cast iron body, details differ like venturi and brass vs steel fittings).
Tillotson was aftermarket, but I do wonder when they came out.
We ran Tillotsons back in the 1960s so had to be before then.