Another update: The 1968 Pontiac Sales Manual has this mention of the insulation on page Q-2. "Special Acoustical Features. Certain 1968 models have added acoustical features to enhance ride, comfort and quietness. For example, the upper-level ventilation system in Firebirds and Grand Prix permits the elimination of door control vent windows, for reduced wind noise. And, with the exception of convertibles, the Le Mans, GTO, Catalina Hardtop Coupe, Executive, Bonneville and Grand Prix models feature sound-absorbing, perforated vinyl-coated fabric headlining to effectively dampen noise.
In Convertible models, special cemented fastening of the top fabric at the center roof bow, plus a tighter seat at the windshield header and a taut cable in the side rail area, help minimize high-speed flap and wind noise in the top. Convertible also have a heavy cotton-fiber lining in the top storage well to muffle out noise in that area."