Re: what is artist proof
Johannes - I think opinions on this vary all across the map. Mainly, my suggestion is that it depends on the reputation of the artist and the actual or potential demand for the work. The artist's proofs should be valued more highly than regular casts, and in my opinion, the higher value should reflect the highest care with inspection and correction of trace problems.
I believe France, historically, was the country most careful with these matters, and it's possible other countries in the developing European Union may have adopted some if not all of the French rules.