Hello.
Just some other notes.
If you have problems with the velvet where the dark slide goes (or where the plate holder slides on to the back of the camera), it is often due to it being matted down after sitting for eighty years. It actually helps quite a bit to take a very soft paint brush and brush it back into the correct form (standing more on end). Also, when doing that, leave the dark slide out for a while to let it relax.
One other note. If you get 5X7 sheet film it can be slit right in four to 2-1/2 by 3-1/2 which fits into these holders. If you get the ortho-litho from Freestyle it comes to about nine cents an image.
I just bought some AP plate holders that had little sheet metal frames to slide the sheet film into. This must have been sold as an alternative for the plates. If you want, I can make detail drawings of these so you can copy them.
-Brian