Scottish singer Sheila McKinley was part of the famous Les Humphries Singers in the 1970s. Found in the WWW.
Sheila appeared with denim waistcoat on promotional photos from around 1975:
She also wore the denim waistcoat while an appearance on German TV-show DISCO on the 26th of October 1974, promoting their single DO YO WANNA ROCK'N ROLL
Here is another set of vidcaps, taken from another video of the same appearance, not available anymore:
