The book explains the early versions as well. Their theory is that the Church approached DaVinci to make a more realistic one than the obvious frauds floating around, which were an embarrassment.
As for their claim of “Shroudie” frauds, I’ll have to find the book and get back to you.
I’ve read it.
Picknett and Prince offer no evidence for any of their fantastic (in the fantasy meaning of the word) claims. There are extant copies of the Shroud made before Leonardo’s birth painted by quite competent artists and they show little to no difference between what we see today. Picknett and Prince just ignore any factual evidence which shows their theories to be absolute twaddle. They just want it to be true that Da Vinci was capable of creating something that would challenge scientists of the 20th and 21st Centuries using techniques those later day experts cannot discern method, medium, or technique nor match the results with any modern method, medium, or technique which meets all the required criteria.