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I would think it would be stamped if it wasn't A made. But that's not dogging the part at all. Have you put it on and noticed a positive change? If so you want to hook me up with some dimensions so I can get my GF's dad to make me one.
The answer to the show and tell is Flight Aids Plenum Chanber. It was marketed in the early 80's by Flight Aids Engineering in Segundo CA. I have never seen a picture of advertisement for it. All I have is an old Rotary Review magazine that did a performance test. Maybe someone can verify my guess. The part I have is missing the Id sticker.
john