There is no mystery in this puzzle, call it like it is! Distributed by Rushmore Photo inc. over 250 Tripl-thick pieces. Obviously a souvenir from the monument. I think I’ve owned this puzzle for a good 8 years, I found it at a Salvation Army thrift store for 49 cents.
I honestly cannot remember if the puzzle was complete the last time I assembled this, but I dropped this in my kitchen the other day and pieces went everywhere, so it is quite possible that I lost that one piece.
Another thing that I noticed just today is that Tripl-Thick (normal for vintage puzzles) appears on one side of the box and on the other side there is a typo. Makes me wonder if this souvenir was bought just outside of the monument gates.