It's actually quite odd, Ive seen one of these before, but it was so long ago I'd forgotten they existed.
The Mustang 'S' actually looks more like a VC-Sports than a VC-Mustang.
Acording to my books, it's run was 76-78, and it was coded V207.
Series 70 MKIV 160cc Eengine, with a G4 carby, and the remote primer setup (also seen on some mustangs) To my knowledge it was not the power plus, but I will have another look tonight to see if i can pin that down more.
The chassis and cowl are that of a VC-Sports, with a darker green paint by memory, and a white dress cowl (as shown above)
I have a feeling the 'S' refers to Mustang "Sports" however that is unconfirmed.