This is something I didn't think existed, a glass theater slide advertising Alice in Wonderland. I thought that these glass 'Magic Lantern' slides pretty much went the way of the Dodo in the 1940s, but here one is (thank you eBay!). I'm not even sure if these were distributed by NSS, but I'm operating under that assumption. This particular slide came out of the Rialto Theatre in Joliet, IL.
These were used by theaters to advertise coming attractions, local business, etc., between features and sometimes between reels. There were also sing-a-long slides from way back when the theater would only have a single projector and it took time for the projectionist to switch reels. At that time these kind of slides would be used to keep the audience entertained until the next reel was ready to show.
There is a pretty good article on these kinds of slide over at LAMP, if you are interested check it out.
UPDATE: I just noticed that the campaign book has a small note on the caption for the A-style half-sheet that says "Slide Same", which I guess means that the image is the same for this slide. No other mention of slides appears in the campaign book.