[blucar]

Phenolic (brown) spacer blocks use to be very common on many engines up through about the early'70's. It would appear that about the time heat riser valves disappeared from smog controlled engines, the phenolic spacer disappeared to.
Ford used phenolic spacers on many of their engine, especially the Y-Block V8's, I seem to recall that the 289/302 etc., had phenolic spacers under their carbs..
Until recently I had a '57 T-Bird 312 CID for many years, it had a large Holly 4 barrel with a phenolic spacer w/vacuum port. I used that port to hook up a stock PCV from a '64 Ford trk Y-Block.

I recently had a problem with a 2001 SBC 350/330 hp V8 w/AFB carb in my 39 conv. The engine ran fine except it was very hard to start when warmed up. The addition of a phenolic spacer solved the problem.