The December '74 Record Plant performance seems to 100% confirm the chronology of Bobby Hunt and Doug Graves.
At the end of the simulcast the announcer reads Mick Fleetwood, John McVie, Christine McVie, Bob Welch and Bobby Hunt.
The September Record Plant performance featured Doug Graves.
Thus, DG worked on Heroes Are Hard To Find and joined the band on tour. At some point he left the tour and was replaced by Bobby Keys. Bob W must have given his old mate a call to help them out by filling in.
I wonder why DG was given the boot.
PS
He later went on to work with Peter Green...
...he dug graves 