Raleigh Brake Fiber Washers
I received in the mail a bag full of M8 red fiber washers (about 8mm inside diameter; 16mm outside diameter). I bought these online on eBay. These are the style used on Raleigh and early Schwinn ("Schwinn Built", not "Schwinn Approved") brake calipers. Sheldon Brown remarks that these red fiber washers are used as buffers to properly space the halves of the caliper and to provide some lubrication while preventing binding.
I used a vernier to measure the fiber disc off a set of Raleigh brakes from the 1960s, and found that an M8 fiber washer is very close. So I ordered a bag from England (could not find a good price on small bulk - 50 or so).
On examination, these red fiber washers from Britain are much like the old Raleigh washers. In terms of hardness, they're the same thickness as a Raleigh fiber washer when new, but slightly harder. They strike me as being closer in size to Raleigh fiber washers, but more of a hardness like the Schwinn-built caliper washers. With a drop of oil, they will work nicely.
Why would you buy such a thing? First, to replace a missing or broken original fiber washer - that's the obvious one. But secondly, sometimes older brake calipers develop a lot of "flex" owing to the compression and deterioration of the stock fiber washer. A new, thicker replacement can sometimes reduce or eliminated unwanted caliper flex. I have fixed two sets of Schwinn-built calipers this summer using exactly that method: replace the old fiber buffer washer with a newer, thicker one.