This is the MAPU flag. Antonio Márquez, 6 March 2000
Green field with a red star outlined in white in the middle. Source is Smith (1975) [smi75c], pp. 340-341 ("Symbols in politics"), where it is identified as the flag of the Party of the Radical Left (Partido de la Izquierda Radical). Smith says that these are real flags and not only party emblems, which may differ in colours when used as emblem or in a flag. Ivan Sache, 6 August 1999
MAPU was created in 1969, and supported President Allende's government. Stil exists as a minor party, not having representatives at the congress. Antonio Márquez, 06 March 2000
From Encyclopaedia Universalis:
The Radical Party split in three fractions: a left wing, which kept the original name and supported Salvador Allende; a right wing, the Radical Democracy, and later, during a third scission, the Party of the Radical Left, which, after having supported Allende's government, joined the rightist opposition.
I suppose the party was suppressed during Pinochet's dictature. Ivan Sache, 6 August 1999