All three of mine have felt: elderly (60's) monorail Arca swiss, Gandolfi variant (uses a metal Cambo back) and my Shen Hao.
Interesting: none of my Arcas of that vintage use felt in the backs, but they all used foam between the bellows and the standards and between the standards and the backs and boards (foam which in my experience isn't really necessary to keep them light-tight).
I wonder whether yours was a felt for foam DIY replacement, or a version I haven't come across - which is very possible as they seemed to make all sorts of variations.
The only Arca things I have with felt in them (not counting the velvet light-traps in filmholders and RFBs) are a sliding back for RFBs on the 6x9 models, and a 'slides two ways' four-on-a-sheet dividing back for 4x5.
Velvet, not felt. If I couldn't use my old plate cameras as an excuse to lurk in the ribbon and lace section of my local haberdashery, something else would have to be found....
Well... there's ribbon to make tabs for lifting things out of fitted cases, velcro tape for securing dark-cloths, safety-pins for everything imaginable...
Not to mention remnants of voile and net-curtain material to use for diffusing light (not
on the light), all sorts of shiny fabric for bouncing it, and just about any sort of fabric for backdrops.
I like my local fabric shop
Peter