First welcome to the site - hope you find the information and help here useful in your concours related efforts
Going to be real tough getting a good idea of what the original insert material was using pictures posted on the web or sent to you.
I assume that you already used the search feature here (lots of information can be found using it) to look at the pictures already posted. Found a couple of threads just using the word "cloth" Unfortunately one with multiple pictures has broken link to those pictures since the author choose to use a different site to store those pictures.
Do have a section of the original material used around somewhere. Its in a box of show and tell items I use for seminars. Most have never seen the cloth insert interior and if they have most of the time the inserts have been replaced. Will look for it though not sure how those pictures will look. The original material has fine thin rows of short nap material. These fine rows form a secondary pattern
Here is a close up that I had on file from a 5F152xxx range example - Hope it helps
