Early Y-Block distributors were vacuum only units referred to as Load-o-matic. These distributors picked up a vacuum signal from the carburetor and varied advance based on engine demand. The Load-o-matic distributors will not work with modified engines! Usually the early distributors have metal tags to identify the part number. When the cap is removed the advance springs are visible. These distributors were used from 1954-1956. 1956 4bbl. cars used a dual diaphragm advance unit.
Beginning in 1957 centrifugal advance was added. The body of the distributor is thicker and the springs are no longer visible with the cap removed. The rotor can be turned and will then spring back to it's static position. Part numbers are stamped into the body . These distributors used a full diameter breaker plate which rotates on ball bearings. The vacuum advance arm is straight. Points cap and rotor will not interchange with the '54-'56 distributors.
About 1959 the ball bearing breaker plate was replaced with a hinged plate which pivots on a single pin. The vacuum advance arm is curved. These units use the same points, cap and rotor as the '57-'59 units and later 289-302.
12127 is the identifier for a distributor. The last two digits indicate actual application.
YEARS PART NUMBER
1957-59 FEK or FEH 12127 Ball bearing breaker plate
1959 B9AF or B9TF 12127 Hinged breaker plate
1960 C0AF or C0TF 12127 "
1961 C1AF or C1TF 12127 "
1962 C2AF or C2TF 12127 "
1963 C3TF 12127 "
1964 C3TF 12127 "
Late and service replacement distributors use the 289 style triangular body with o-ring seal.
Not all of these numbers may actually exist.