Yeah, you’re right about TTMAR. I never thought about that. Wasn’t ad-libbed though — and very different in tone.Woody Allen did a feature mockumentary in 1969 called Take the Money and Run (available in full on Youtube btw). Fellini also did one in 1970. Non-comedy versions go back to the 30s. Although Spinal Tap is certainly the best example of the genre, and the one that influenced the most copycats later on.
Best bit of that film...