Well, Mark Evans used one too - so it might have been something bought as a stage backup instrument or something?
I remember reading somewhere that they only travelled with one spare guitar (telecaster) and one spare bass in the early days.
It can be seen in the 'dog eat dog' promo vid from let there be rock.
It's a reverse.
Here's the video, most likely the same bass but Cliff already in the lineup. This was from some TV show, probably from 1978.
EDIT: It's Mark Evans on the video too, not Cliff. I must have confused it with an old "Let There Be Rock" promo (the church thing) where Cliff is playing a Ripper, or miming with it.