Matteo,
The most significant difference is the timing cover. On the 1927-28 models the oil sight glass is built into the timing cover casting. On the 1929 models the oil sight glass is a separate bolt-on item supplied by Enots.
Engine serial numbers for 1929 would be over the twelve-hundred range, but no one really knows what the cut off point for a 1928 versus a 1929 engine serial number would be.
Also, if I remember correctly, there is a slight difference in the cylinder barrels around the lower area of the valve chest (more enclosure at the bottom edge?) Daren, correct me if I am wrong. The cylinders are interchangeable between 1927-28-29.
-Doug