I am with CS here.  However I would go for it!!  Bite the bullet and see if you get lucky!  Then again, it isn't my money... haha!

It's a flat pack.

I would do some research to see if there are any varieties to be found, and if so, then consider getting it.

I ordered an "unopened flat pack and was very luckey. No milk spots. No marks on the coins. None of the boxes opened so each coin was in its spot. No interaction with the other coins. Those are far and few between.

I thought these came in an envelope encased by cellophane. I wouldn't worry about the paper envelope as much as I would the cellophane. In fact, if the cellophane has been opened, it would be very difficult to re-encase the coins back. Without pictures, it would be hard for me to make an opinion.

Stumpy,I do not know how to tell if the envelope was opened and resealed.  I would do some on line research for a laboratory that specializes in paper or adhesives and ask them if it is even possible to answer your question.  As far as what to do with it, it is a good dilemma to have.I like your idea of the one hundred year anniversary.  That way you always have the option of opening any time you want.  Or you can open it now.  There are no right or wrong answer.Sorry I'm not more help.Stay warn,Mr.B

look near the opening of the envelope. if it was resealed then their would be a little residue near the side of it that would be yellow. 

If my buy something unopened,  you can be sure I would open it. Only think I bought recently was one if those BU rolls of wheats with a reverse showing. Tough decision to buy unopened and tough to stay that way. If the dealer is good, reputable, and has nice reviews buy something unopened. 

I would say they are quite common, whether legitimately or not, I can't say. I have had a few times where unmarked US mint envelopes for proof sets have had to be opened at the show so a dealer can accurately price them. I would advise you to buy the coins, not the holder, or in this case, the envelope. They should be common enough to find without milk spots, even some toning.

@Golfer
If my buy something unopened,  you can be sure I would open it. Only thing I bought recently was one of those BU rolls of wheats with a reverse showing. Tough decision to buy unopened and tough to stay that way. If the dealer is good, reputable, and has nice reviews buy something unopened.