The notorious "Rainbow" concert bootleg indeed was recordeed in London in January 1974.
The dependable NME (New Musical Express) says in its February 2, 1974 number: "Wonder's Rainbow gigs last Thursday were recorded by Tamla Motown for subsequent release as a live album, and Stevie has already been in the studios to start mixing the tapes." and in their August 23, 1975 number they write: "Several projects have been mooted; his Rainbow gigs last year were recorded for possible release, but it is understood the tapes were not technically up to standard."

description by Thomas Winberg