I am sure most will have realised this before now - but it took me a couple of years for the penny to drop.

I have a number of field cameras (9x12, 13x18, 18x24 and 30x40cm) most which came with book form (opening in the middle with releasable central metal plate) plate holders. A few of these came with film sheaths as adapter to sheet film - but many without.

Although sheaths work excellently, there is always the problem of checking and adjusting the ground glass position as the raised edges of the sheaths do increase the film plane depth in most holders by around 1 mm.

Eventually I have realised that these book holders do not need sheaths, as the internal mahogany framing is less than the nominal size. This means that film can be "placed" into a precise square "hole" from the rear. All that is needed is a stiff plain cut out which covers the film and is pressed against the film by the single or double spring on the central metal plate.

After this discovery, I have used the metal film sheaths I have in a reversed position as a temporary (expensive, but a forgotten expenditure!) measure. Experience in a changing bag says that the system would be easier/quicker with specially made "pressure plates" which fit the "square hole" exactly. There is some difficulty in ensuring the pressure plate has "fallen down" with these slightly indersized sheaths.

I also note that 5x7" sheet film can be used in the same way with the 13x18cm book holders I have - there is enough film overlap.
The side with central latch should be done first and it is easy to mistake negative side of the film when working from the inside out, so to speak.

I will post again when I have found a suitable replacement for these, rather heavy, steel sheaths.