Number Line aka Printers Key Harper Collins Explanation

Many thanks for your email sent via HarperCollins.co.uk, and apologies
for the late reply.

With each successive reprint, the publisher needs to instruct the
printer to change the impression number, and the theory is that they're
less likely to make a mistake if all they're doing is removing the
lowest number rather than changing (and introducing) a new number each
time.

So, with this arrangement, all the printer has to do is "rub off" the
outer number that's lowest in the sequence. And by changing only the
outer number it means that the fewest possible changes are made to the
page of characters, which means the smallest possible charge to the
publisher. (In the days of hot-metal printing, where each character was
a metal block, all the printer had to was literally pick out the
relevant blocks from the "sheet" and then the stack of blocks - which
would have been laboriously laid out when the page was first set up -
could be inked up for the reprint.)

I hope this helps.

Regards,
HarperCollins Publishers UK