Guys, think your way around this problem. Yes, I use plastic sheet protectors, and they're a pain. Especially when you want to remove them.
I saw the problem differently, and solved it a little differently. I took my then sizable pile to work one day and made 2 copies of each letter. Then I brought them back home and filed the originals in a binder and put it in the safe. Then I punched holes in the first set and bound them in a binder in serial number order, regardless of the frame size. Then the 2nd set went in a binder by caliber. For a number of years the binder in serial number order traveled with me to gun shows, and otherwise resided next to my easy chair (the one with the laptop.)
Several people had pointed out to me the worst mistake I could make was carrying the originals to gun shows. They seem to get lost at an amazing rate. Because that also happened to copies, I think they're doomed. The exception was when a gun was "pre sold". Then the gun and the letter and anything else pertinent went in a box and taped.
Copies are great for making notes on. Don't do that on an original.
Another question that might be considered thread drift is selling the gun with the letter. Do you feel paranoid and not want others to see your home address? That one struck home a while back (I'm not hard to find) when I was aimlessly wandering a gun show. There on a table sat a very familiar looking handgun. Just like one I sold a few years before that. And right underneath it was a factory letter. Addressed to me! Ugh. I've bought guns that way in the past, and I've bought guns with the name and address cut out. Probably with an exacto knife.
With a copy of the letter, you can do that without any damage to the original. The classy way is to give the buyer the copy, then promise (and follow through) by sending him the original in the mail. The very next day!
Nailed to the outhouse wall explains the new aroma in my gun room! I never expected that.