They were airbrushed on top of a black & white photo print. In the days before digital retouching basically all marketing material was airbrushed by an artist. My brother used to practise that as well. I am a digital artist myself.
I do like the airbrush look.
BTW none of those clips left in my car and engine, so it is good to know about them.
But in that photo / illustration it looks as if the coil in the vacuum pipe would be right below the rear carb and too high to have a clip on one of the tappet cover bolts?
FWIW my vacuum pipe had been pulled straignt by someone in the past. I broke it at the dizzy end and bought a replacement from SNGB and it is much shorter. So short that I could not make a coil like that, but it’s there in the car now between the front carb and the dizzy.