Got an old arrangement where the 1st bone part is marked "FIBRE" and then later "Open", which leads me to believe fibre is some kind of mute. Anyone know for sure?

Compressed paper straight mute (i.e., Ray Robinson or Stone Lined).

Back in the day, there was another sort of fibre mute that had a sort of "golf ball" kind of surface that was sold under the "Micro" brand. I don't think anyone makes anything like that any more, but I see used ones for sale now and then (there's actually [url=https://www.ebay.com/itm/266356844267]an NOS one for sale on eBay right now). A little different sound than a Stone Lined, but also not nearly as durable.

Thanks!