I have gone through hundreds of 1968 proof sets, but first time I have come across this style of box. It is the style of the 1967 SMS box where it opens on both ends, but it is wider and shorter. I know it is a 1968 Proof set, but I thought it was cool enough to share. Any help on finding information on proof set box varieties would be appreciated as well.
My '68 set can be seen in this thread*, along with a lot of sets since. Hard to tell from the one picture I show, but I think it opened on the long side, not the ends like yours. * https://www.cointalk.com/threads/coincorgis-u-s-mint-proof-sets.365559/
I do not have my set handy to take a picture of, but I will try to get one up in a few days. I love your penny set BTW.
Probably, since most of the 68's and as far as I know ALL of the 69 - 72 sets are the other style, and this sets box is similar in design, if not size, to the 66 and 67 boxes.