Please do not write below the line

Likeliest situation is all their stationary destined for send outs have the line; and in situations where the line serves no purpose it does no harm to leave it: so there is little use in having additional process around completely blank stock.

They could have told him that though if it's the case, and the mystery would be solved. But there obviously wasn't any desire (or more charitably - time) on their end to really look into the reasoning or even understand the question.

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Edit: I will share what I think is a nice a little counterpoint story here, from a business that is clearly still interested in understanding. I sent Lego an email a while ago:

  I'm just wondering if you're able to tell me what the
  tune is that the Lego Primo musical camel plays in set
  number 2007. It's a set from 1998. We have the camel and
  it plays a nice tune, but no-one seems to know what it is! 

They replied a day later:

  Thanks for getting in touch with us. 

  This is a really really really and I mean really interesting
  question you got there for us. I have checked with all the
  resources I have and come to a possible conclusion.

  The Musical Camel – which in Denmark actually is called
  ‘PRIMO Dromedar'. 1st theory is that, One DUPLO-designer
  says that the melody was composed by the designer that
  created the camel but no one remembers the name who created
  the Musical Camel. Another thing is, one of the engineer once
  had a musical box that had the same melody but he is no
  longer with us anymore and cannot provide us the answer.

  I am so sorry that, at the end of the day I cannot provide
  you with any name to the title. But I hope the facts can
  make a good story for you to tell your friends.

That is an amazingly competent response. Every company should aspire to that kind of depth, if a toy company can manage it.

I cannot find it, but I once sent an email to johnnie walker to ask for the music for what is arguably the best ad I ever saw (except for think different). They correctly answered, though the answer was the music is original and copyrighted, and thus I could not get it.