When asked for a cigarette, Mannix says he hasn't smoked since July (1971).
Joanna Pettet was here eerily a dead ringer for Sharon Tate. Incidentally, they were friends, and Joanna said to be one of the last persons to have seen Sharon alive.
When discussing her career travails, the character Cindy Warren alludes to the recent cancellation of The Ed Sullivan Show (1948).
Button Gwinnett, whose signature on the last letter he wrote was the item of value for which Mannix was being pursued, was not a fictional figure; he was actually a signer of the Declaration of Independence who died in a dual shortly thereafter. A county in suburban Atlanta is named after him.