For the 1925 season, in order to be eligible for the championship, manufacturers had to compete in the Italian Grand Prix and the Grand Prix of their country of origin.
For 1926 and 1927, no scores were dropped and eligibility was gained by competing in at least two Grands Prix in addition to the mandatory Italian Grand Prix.
For 1928 a seven-races schedule was planned with mandatory participation at three rounds.
However, only two races were effectively held according to the AIACR regulations, so the title wasn't awarded.
[2] A similar situation happened in 1929 and 1930 seasons since only one race was held to the AIACR regulations.