They mentioned a desire to protect the Constitution from Allende, and supported the military's intervention to do so. Pinochet did not become head of the military until August 23rd. Frei had been calling for Allende's ouster for several months by that point though. He quickly endorsed the Chamber of Deputies' call on August 22nd, and supported the newly appointed Pinochet as the one to lead the military response.
I'll ask you again: Did these letters mention anything about Pinochet's alleged desire to protect the Constitution?