Here's a bit of comic book history I found in my files and scanned to share with you. Between around 1966 and 1968, if you sent a letter in to any of the DC comic books featuring Superman, you might get back a copy of a form letter. I sent a lot of letters so I got a lot of copies and sometimes, as you will see, someone in the office would write in a little personal note.
I believe the form letter was written in early '66. There's a reference there to Supergirl appearing in The Brave and the Bold, which happened late in '65 and there's mention of a forthcoming Superman cartoon show that debuted in the Fall of '66.
In any case, I believe this particular copy was sent to me some time in 1968, which means that some of its info was outta date when I received it. The handwritten P.S. talks about a change in the size of the original art that was made in 1968.
I'm not sure who wrote the P.S. but a likely candidate is E. Nelson Bridwell, who was the assistant to Superman editor Mort Weisinger at the time. Nelson probably authored the entire letter, too. (By the way, I don't know what he meant about their artists drawing on a special type of paper unless he meant that they used decent art board instead of the kind of paper that a kid might have available to draw upon. Artists then as now were drawing on two-ply Strathmore or some other kind of bristol board.)
That's about it. Click here to load a copy of the letter as a PDF file or right-click to download it. Hope you find it interesting.