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.
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Peter