This is what we had before PowerPoint.  People made professional slide presentations, and to sex it up, you could use these.  There are slide holders that have a panoramic opening across the middle, some with an upper 1/3rd, some with an upper 1/4 corner opening.  There are also blanks, that is opaque pieces of plastic to give a black screen.  

I believe these are GEPE, the only place I saw the name was on some of the blanks.  They could be some other make, or a mix of makes.  I'm selling these as 'used' because 2 of the slides have images, but almost all are probably new.  They have anti-newton glass, 1 of which is cracked (I saw 1 crack, I might have missed one or two other cracks).  Many of the blanks have writing, and they are from a variety of makers.  

Note: these take standard 24x36mm film slides, the film does not have to be cut down to mount it.

17.5x11.7mm opening in 1 corner: 45 slides
34.9x11.9mm panoramic opening in center: 12 slides
23.4x17.2mm centered horizontally, off-center vertically: 45 slides
Blanks: 30 slides

Extra: 3x unused standard-size Perro Color slide mounts (chromed)

These will be well-protected for shipping, I have sold glass slide mounts before without incident.