I didn't spot this thread before, as Umut says I was away on a workshop
One of the major suppliers of changing boxes was A.Adams & Co who were selling them for Plates and Cut film by 1927, plates only in 1921. Film packs seem to have been far more common. Looking through a 1927 BJPA theres not much cut film available from any manufacturer although film packs had been available for some time, Ilford offered a choice of 2 cut films and 17 different types of plates (excluding X-ray & Process).
I bought some 5x4 Plate holders last year quite a few had pressed film inserts and were made to be dual purpose but one a pre-WWII Graflex holder has thin plain metal sheet with folded edges similar to those often used to covert 9x12 plate holders for film. You might be ale to make some yourself for the septums. they are very thin.
Nice camera BTW I'm jealous I just missed out on one a few days ago. I supply shutter curtain material for these older Focal plane shutters, the last was 6ft x 8", but don't have a British relfex camera yet . . . . . . . .