Can anyone confirm/deny Grouse's comment from the article?
"It's unlikely that a single set of metal plates would survive multiple printings. They wear out pretty fast. Instead, this would be used on the film negative used to expose the photosensitive plates. Of course, now people are printing directly to the plates instead, skipping the film step."
http://blogs.msdn.com/b/oldnewthing/archive/2007/04/10/20657...
For xerographic (and other chemical process) prints, what you say is probably true. For relief printing (think Gutenberg), this convention seems reasonable, though you'd have to grind, not scratch, away the extra number. I don't know enough book history to say whether this convention is old enough to have started from relief printing, but that's the only way it makes sense to me.
"It's unlikely that a single set of metal plates would survive multiple printings. They wear out pretty fast. Instead, this would be used on the film negative used to expose the photosensitive plates. Of course, now people are printing directly to the plates instead, skipping the film step." http://blogs.msdn.com/b/oldnewthing/archive/2007/04/10/20657...