Prototypes and engineering samples are often used by the sales and marketing team to develop advertising before production starts. No one can wait for the production pieces to be first assembled in the factory before creating the ads and owner's manual pictures etc. However the designs have already been tooled and the production lines are close to running so it's not like the cartoony drawings they used back then for advertising were what drove the eventual design of the bike. I would really doubt that.
I'm not an advertising expert but is it possible that the drawn and colored ads that were used back then were just easier and cheaper to reproduce than actual photography? I lean more toward
@rustjunkie and
@GTs58 thinking on this. There's tons of things you can draw that can't be produced. Businesses don't operate like that and stay in business long.