Proof that the seven days of Unleavened Bread for the Quartodecimans extended from the 14th until the end of the 20th day of the first lunar month is established from records provided by their offshoots, the quasi-Quartodecimans of System D.
The most important source for their view is found in the records of Anatolius of Alexandria.
To his words we can add the statements provided by the Audians and several bishops representing assemblies located in different parts of Europe.
Anatolius of Alexandria
Like the Quartodecimans, those who kept System D observed the 14th until the end of the 20th for the seven days of Unleavened Bread.
The most famous advocate of this system was Anatolius of Alexandria (c.230-283 C.E.).1