So you sandwich these between the negative and the photo paper. Right?
And, because they are deep red, the paper isn't exposed around the edges.

Yes, I found them in a box along with some contact frames.
It all makes sense, now.


BTW: Just a fun fact. It is illegal in Pennsylvania, where I live, to even set foot in a theater projection room with a reel of nitrate film in your possession unless the projectors are all outfitted with certain safety devices to prevent the spread of fire and other fire safety systems to protect the patrons in case of fire. There are laws to the same effect in most states, as well.