I've met a few knowledgeable guys that didn't know they even existed! They dropped them for a year or two around 1921, but apparently realized the brackets weren't getting the oil they needed, so they reinstated them. The early ones take a rectangular felt, and the late ones use a round felt. Kind of important from a lubrication standpoint.
Larry thanks for bringing this up I discovered the felts on my 25 when I had the controller shaft assembly out for a clean-up, inspection and repaint. They were in a petrified state, so I soaked them in kerosene,then lacquer thinner, then finally some light oil,this process brought them back to life. As you know there is a small hole in the frame located between the attaching bolts so one can oil these. Mine were all but invisible being plugged with dirt. As mentioned, I'm sure there are some here that don't know these felts even exist.
The brackets are included on the lube charts, but my guess is that most people don't know what's inside them.
I use a couple pieces of Green scratcher for a felt in mine.They hold up well and keep an oil film on shaft.